You need an accelerator and the minimum you should get is an ACA 030. Or you could check Amiga marketplaces in forums or Amibay for faster turbo cards. You would need at least 16mb ram in my opinion but 128mb is good to have.
I advice at least a a basic buffered ide interface and a 3.5" harddrive which you can use next to your Amiga. and/or you can get a cf card slot or an sd slot.
In my opinion a proper power supply (which you can use to also power the 3.5"hdd and a cdrom) that's not older than 5 years is very important because you can not trust old ones. (I did and lost my A4000T. 10 years without it...)
A compatible PCMCIA cabled or wireless network card.
You will need a scan doubler flicker fixer if you plan to use higher resolutions and standard monitors.
You should upgrade to latest kickstart (3.1) and get yourself AmigaOS3.9
These are the most important things.
The rest is getting familiar and fine tuning things and tweaking things to suit your taste.
If you want to have rtg, you can either get (or make) yourself an Amiga tower case and buy a Mediator and a compatible graphics card... or you can get a BlizzardPPC with a BVisionPPC although they are very hard to come by and very expensive.