My modest opinion is: you should consider getting an emulator instead, such as WinUAE and/or Amithlon. Performances are far better on a modern PC with an emulator than you would get with an already outdated Amiga.
It is faster in raw cpu perfomance, but it still sucks when it comes to chipset emulation. The gfx does not run as smooth, the sound is either choppy or lagged even with the most modern pc. When it comes to classic games, it is just not an option if you are picky about such issues. It annoyes the hell out of me to play a game with sound lagging some ms behind the graphics. It also annoyes the hell out of me when some background process makes the graphic jerk. Winuae is however a good option if you want to for example use cpu intensive software like 3d rendering programs and similar. The main advantage of AmigaOS is the fact that it is lightweight and only needs a few seconds of boot time. With winuae you lose all that due to the fact that it is hosted ontop of another OS.
An a4000 is a great choice in my opinion if you have the cash.
