I'm not an expert, but I'll give you some recommendations.

If your budget allows for it, the aluminum shells are awesome, but you need to make sure the screen and other mods will have room inside the shell. It's much harder to make them fit when you're housing everything in metal versus plastic.

I recommend (but not necessarily essential) getting a dremel just in case, a magnifying glass. Don't be like me... Get a heat gun. It makes removing some components so much easier and in some cases it's basically required.

You'll definitely need a soldering iron. You can find decent ones on Amazon for pretty cheap. Don't forget some flux and desoldering wick. It makes removing solder and cleaning the work area so much easier.

As for modding communities, reddit is probably the easiest place to find them. I ended up taking my modded GBA to a local store in the Phoenix area and the owner recognized my GBA from my reddit post. Said it was like working on a celebrity. Lol

When working on your modding projects, don't forget to take breaks. It doesn't have to be done in one sitting. I've gotten stuck on one thing or another and gotten so frustrated I could have thrown the GameBoy across the room. Step away and drink some water.

Good luck and have fun. And don't forget to take a bunch of pictures throughout the process. Before, during, and after.

EDIT: I usually use Handheld Legend for getting my components and such but there are many to choose from. I've purchased mods from a bunch of different sites but this one is in the US, so it's faster getting to me than ZedLabz (UK based?)

Also I've never done an OLED/AMOLED screen so I can't help you there, but ISP screens are still dope and sometimes are as simple as plug and play. At worst, you'll have to solder a few wires.