Help choosing first mechanical watch

Have you tried any Omega, GS, and IWC on wrist, in-person?  My guess is that the moment you put them on wrist and have the opportunity to feel what would wear best for your particular wrist, you'll simply KNOW which is the right one, like this...

I concur with @HotWatchChick69 .  It seems like you have a solid amount of research into what you want in terms of features and specific brands.  Each brand you listed makes quality watches.  You need to see the watches in person at this point.  

check out longines heritage collection, or spirit colleciron -Zulu , Breitling. 

this is my recommendation 

I personally feel the IWC doesn't live up to Omega and Grand Seiko.  I own a beautiful Seamaster Aqua Terra but when I put on a suit it is not the watch I reach for.  I think it is a bit too sporty but if I change the strap to leather I can pull it off.  The Omega DeVille Presage 39 mm fits the bill and hits your price point brand new.  

In Grand Seiko the SGBW285 37.3 mm gives you all of the styling they are know for.

Hello,

I suggest you check our Zulu time, the latest from Longines and it fits you budget.

https://nomos-glashuette.com/en/metro/metro-date-power-reserve-1101#anchor:specs

The new Mark XX is also super nice!

I disagree that Omega is out of the question.  Based upon the OP wanting a multi situational watch, I would say that the Seamaster Railmaster could fit the bill perfectly.  List is $4900, but I’d bet a hundred that most ADs would discount to make the sale.  I have one with the brushed steel dial, and it is a total strap monster.  The Railmaster is under appreciated, generally. However, using quick change straps it can go from a beach watch on a rubber strap to a dress watch on an ostrich in under a minute.  I use it like this, and thats the role of the watch in my collection.  I’ll bring it on a cruise with a canvas strap for the pool, and either an alligator or ostrich for dress up.

Your last comment about Tudor bringing some great value to the table has some merit. While they might not be as fashion forward as other brands, I think the designs will stand the test of time and would make for a great starting off point for a collection if you decided to go that way in the future. Pro tip: for some reason, they always look so much better to me in person than in the pics. Go try some on at a dealer if you can.

A ridged bezel with a suit?  That is so uncivilized.

What is your wrist size? What activities do you envision yourself doing with it? Also, what other features are important to you?  This can help us advise you further. For instance, how important are water resistance, in house movement, legibility, and color to you? From what you're describing, it sounds like a do it all watch. Like @HotWatchChick69 said, I highly recommend you go try them on in person.  Some brands like Longines know that they don't have a broad host of retailers, so they will allow free shipping and returns so that you can try stuff on.  Tudor has a many models that would fit, but the Omega aqua terra line, Longines Spirit line and IWC mark lines fit your preferences too.

Also, there are Aqua Terras not much more.  I think if the watch is a real home run it’d be worth it to wait and just save up for it.  It really comes down to the person and watch.  That’s why I think the OP should visit an AD at this point and see what works for them I the wrist.  Plus, most ADs would be willing to work with a customer and you might get them to agree to a price consideration.

I would look towards Longines and their Conquest line. It's a classic style that can be used everyday, but is also fits well with a suit.

just adding one to consider you probably know but it will check your boxes 

BB36

Since you like Omega (one of my personal favorite brands), have you considered something pre-owned from them?  For example, I picked up this Omega SMP 300 (Ref # 2255.80.00.00) with box and papers, in excellent condition, for $2,800 CDN in 2020. The value may have increased since then, but at current exchange rates, that $2,800 CDN works out to approx £1800, so even with any potential appreciation, would be well within budget. From my perspective, it represents great value for a versatile chronometer rated automatic from a respected brand. Even if this particular model isn't to your liking, I'm sure you'd be able to find something that would interest you if you're willing to consider pre-owned.

Tudor Black Bay 36 (I like the blue dial but many options). Buy it on the bracelet and get an aftermarket strap that suits your taste.

How about a Cartier Tank Must XL? Youreterrific made an excellent YouTube video about this watch. It's a dress watch on leather strap, but because of it's size (it is a large watch), you can also wear it casual, at least I would do 😊  

Tudor BB36 in black? I think it should be a very solid choice as a GADA watch, can easily dress up or down depending on the occasion

With that budget and those brands, probably your best bet is either 

1. neo vintage omega. Do a search on Chrono 24 and you’ll get a bunch of SeaMasters for about 3 grand from the 2000s. 
2. IWC spitfire. You should be able to snag one for under 4K and it’s beautiful. 

Most crew members already came up with very valid options, imo. I would support the idea of looking out for a 38mm Aqua Terra that you like and, as always, try it on! If the slight bump up in thickness doesn't bother you, this might be versatile enough to get you through every social situation imaginable, yet special enough to not make you get bored after a few years.

When I look at the Grand Seiko website in my budget the SGBW285 doesn't come up.

The main options I am seeing are the SGBW285, SGBW253, SGBW257, SGBW255.

I am a big fan of the Aqua Terra in Blue, but I worry a bit about how well it will wear with a suit.

Do ADs really move on price to get a deal? I had, perhaps ignorantly, assumed that the list price was the final price.

An Omega Boutique will not move on the price of a purchase under $10,000.00.  An AD that is not a company owned store will do what they please in order to sell a watch.

The price can be negotiated.  In an overheated market, less so.  But if you are nice, and ask nicely, lots of things are possible. A year or two ago you could, with the skill, get up to (and this is the highest and not every day) a 20% discount.  These days, don’t expect as much.  Omega has righted the margins for ADs within the last year, and the watch market has been very hot.  But I feel like asking for a 10% price consideration is still a reasonable ask.  Be ready to buy right then if they say yes, because if they feel like you are shopping their offer elsewhere it will disappear before you get out the door.  So yes, I feel that if you make a good impression and are ready to make a deal right there then the right deal can happen.  Remember also, if a dealer treats you well and that turns you into a regular customer that buys more watches every one wins.  Every AD is different (unless you go to a corporate store), so visit a few.  Sooner or later you will make a connection and then things get great.  I am very happy with my AD, and they get my repeat business.

Agreed! An Omega boutique will NOT negotiate price.

Thank you for all the comments and recommendations so far. I appreciate the guidance.

I have gone and tried some watches on today. I've attached some photos below.

I have a question about sizing. My wrist size is 18.1cm. What watch size would people typically recommend for this?

I really like the Omega Aqua Terra. I tried this in both the 38mm and 41mm sizes, and I am unsure which I preferred, I think I am leaning towards 38mm.

I think the Aqua Terra blue face was my favourite.

I also tried the Omega De Ville Tresor in 40mm. Which I really liked 😍 But I do wonder if it's marginally too big.

I also tried on some Nomos watches. My local dealer is currently doing 25% off on them, so this is an interesting proposition.

I like the Nomos, but I worry that they are not a classic design and that I am more likely to get bored with them faster than I would the Omegas.

Aqua Terra - Blue 38mm

Aqua Terra - Black 41mm

Aqua Terra - Black 38mm

De Ville - 40mm

Neomatik - 41mm?

Zurich - 40mm?

Orion 38mm?

My wrist size is 18.1cm. 

as I say in my latest reply, I’m not sure what watch size people typically recommend for different wrist sizes?

I have an 18.4cm wrist and generally wear between 38 and 42mm watches. The way a watch wears is all preference, but some watches wear larger or smaller. For instance, the Nomos Neomatik and Orion probably wear larger than 41 and 38 respectively due to the small bezel and lugs.  They would be referred to as “all dial”. The Aqua Terras on the other hand looks like it would wear more true to spec.  Imho they all seem to fit you, but I have found that whatever feels natural on first impression is usually the one that I enjoy long-term the most.

I have now also tried on a Grand Seiko 😍

I think it's my favourite dial/look of them all, but it is quite thick.

37.3mm