To get back on topic here..............................
Ive had a GP100 like yours for a few years now and am very happy with it.
First thing i suggest you do is a lot of dry firing, i mean a lot like thousands of times. This will break in the parts and get you used to the double action trigger. I would sit down and watch some movies and just click click click click. Never used any snap caps and mine works just great but if you are worried pick up a set.
I did get a wolf spring kit which i switched out the main spring and return spring to a lighter one but found some primers wouldnt strike and then the trigger wouldnt return all the way when shooting double action quickly. Again do lots of dry firing.
The only things ive really done to mine is to take out the trigger and contour the edges for double action and smooth out the edges of the hammer where my thumb contacts it. Also i took out the front sight and flattend the face of it so its more like a target sight. I did change the grip too. I dont like the current factory grip so picked up an old pachmayer compact grip which i just love unfortunately they dont sell it anymore but the current one is decent too. I found the factory grip has a bump that shoots pain right into my palm which i did mod but like the pachmayer grips better.
As for gear, if your not planning on action shooting or PPC you really dont need anything. Mind you i do both with mine so i have a loading tray and a bunch of HKS speedloaders which i got cheap and learned how to use them properly or would have bought the Comp II or III ones. HKS requires a turn and click to release the rounds as the Comp II and III require just pushing in the cylinder.
For a holster i have a safari land model which is made of Kydex. I also got it cheap but it is a very smooth drawing and comfortable holster with good retention. Mind you since its unlined it scratches the barrel some. Im past caring about my revolver being shiny and new looking so dont really care. Each scratch is a badge of honor.
I also reload and got started with a Lee 3 hole turret press which i bought used and have made thousands of rounds myself. It is a reliable press but not as fast as others. When i first got it i didnt shoot as much so it wasnt a problem but now that i have 2 kids and shoot a lot more competition i would like something faster but i can still crank out 100 rounds an hour. I dont use it for long sessions but make a box or two when i have the chance and finished ammo slowly piles up. I do like the fact its very simple and checking the powder charge is easy and there is really nothing to mess up. Its a great first press and takes up very little space.
There you go thats my personal experience. Not sure if i mentioned do a lot of dry firing. One more thing i will add a friend of mine traded my GP100 for his 686 for a month when we swapped back we both had the same thing to say, didnt like the trigger!
https://imgur.com/Ks8hOOB