Forget about OS3.9 and all that complex stuff. You just need a basic 3.x install and a game loader and you are good to go. (Also, OS4 is for PPC Amigas. Forget the most about this version)
If your choice is between an A500 and an A2000, get the A2000, because it has more expansion capabilities than the A500. I agree: you shouldn't buy an Amiga, and a 1200, although more easily expandable because of availability of stuff, is not necessary. Let's remember AGA games are a very small percentage of the Amiga games catalogue and, if I was you, I'd forget about Doom or Sim City 2000 (both games you can run perfectly on your PC using DOSBox)
In any case, I applaud your interest for this old shit at your age
. Would love to hear what interests you about these dinosaurs of computing...
Let me tell you you are in for a bit of masochism, but after you learn to set things right, the gratification is high.