For me, the Amiga is not about raw performance - then I'd use WinUAE on my PC which is clocked 500x higher than a low end Amiga. Pretty much all games run in a plain non-interlaced PAL/NTSC mode, and if my monitor had a SCART input I think that would suffice on the A600 for productivity and gaming. On my laptop I can set the screen to 50 Hz, enable Vsync and then WinUAE actually feels quite 'right'.
So one can achieve this feeling with very little money, however if you like to toy with exotic hardware, then an A4000 with 060/PPC accelerator, gfx, ethernet and the lot could be the right thing. We can't tell you what the right thing is.