The main dimension that you don't want to mess with is the height. If the height is a lot smaller, it won't wind evenly and could cause more wear. You should also never get a mainspring that is thicker, although a slightly thinner one could work. Since I'm assuming you don't have mainspring winders, you also want to make sure the diameter of the mainspring is smaller than your actual barrel so that you can push it in.

The one that you found seems to be a really good match, so I don't think it would cause any issues. I wouldn't worry about the .02mm difference in height and the slightly shorter length will affect the power reserve, but probably not by much. Good luck and have fun!

I'm saving up for a Speedmaster so I've been having this debate in my head for a while now. I think the Hesalite looks better and personally I don't have any concerns about damaging it. However, I prefer the polished bracelet links on the sapphire version and I also really want the display caseback. If I had to make a decision today, I would probably go with the sapphire, but I'm going to try them both on before I make the final purchase.

I always clean the screws. Honestly, I doubt it really makes much of a difference, but that seems to be standard practice.

As far as keeping track of the screws, whenever I remove something from the movement I place the screws for that object next to it. Once everything it disassembled, I take some pictures of the disassembled parts so that I can see which screws go with which sections of the movement. It helps to have a tray with some dividers so that you can keep related things together.

Once you do it a few times, you start to get a good idea of which screws go where. It's actually not as hard as you would think. Calendar & Automatic works tend to complicate the screw situation which is why I would recommend avoiding them at first. 

Another update: I've been playing around with my Geiger counter some more and discovered something interesting. While the reading from the dial side is around 1800 CPM on my Omega, the back of the watch only measures about 200 CPM. The movement and the case back seem to block most of the radiation from reaching your wrist.

I had also heard the advice to start with pocket watches, but I chose to go straight to wristwatches since I have little interest in pocket watches. I think watching watch repair videos prepared me very well.

As far as brands, I really like Hamilton because they tend to have simple movements and can be had for reasonable prices. Any brand will probably be ok but stick to 17 jewel hand wind movements with no calendar works at the beginning. Also be aware that any watch with lume from the mid-60's and before will likely be using radium, which requires some extra precautions. Best to avoid those for now.

My best advice is to take lots of pictures when disassembling the movement. Also, don't get discouraged if a part goes flying across the room and you can't find it, that's part of the experience 😂. Have fun!

My Geiger counter arrived today and I can confirm this thing's still hot! It's a little higher than I was hoping, I would have liked to see it under 500 CPM. I'm guessing the majority of that is coming from the hands since there is quite a lot of lume on them. I think I'm going to replace the hands, even though that will probably anger some Omega collectors. There're pretty beat up already, and that should hopefully lower the radiation level.

On another note, although I thought this was the first radium watch I would be working on, the Geiger counter has revealed that this is not the case. I've got an old Hamilton that I used for practice which is reading at 1800 CPM. I didn't take any precautions when I worked on that one since I assumed it was Tritium, so I guess there's a chance I've got some radium in my bones now...

Oh well, there's nothing I can do about it now. I guess the moral of the story is if you're planning on taking old watches apart, you should probably invest in a Geiger counter.