IMHO if you want to play Quake, just get a modern-ish PC, it's going to be a lot cheaper and provide a much better experience. If you want the really authentic retro PC gaming setup look for something like a Pentium-200 with a Voodoo2 card and a 19" CRT, that was my dream system back in the day. The charm of the Amiga is all the awesome classic games, most written for the A500 but they'll run on the other models too. Personally I'm a huge fan of CRT monitors, the classic stuff just looks wrong on a modern LCD but CRTs are getting hard to find.
Something you might consider if you're on a budget is something like the MIST FPGA based retro computer or other similar platforms that support the Minimig core. It's pretty reasonably priced compared to a real Amiga, under $250 I think and it's all brand new hardware so there's no messing around with vintage stuff in unknown condition. I personally like tinkering with old hardware but it's not for everyone and it tends to not be cheap.