More or less same advice as the two above.
A500 1MB chip, gotek, some game/other disk images and maybe a joystick and see if this is something for you. If you buy it working and tested, this setup doesn't need much maintenance and relatively affordable. Workbench will be a bit barebones but disk games will run fine.
With some more cash to burn an A1200 with a CF card for a harddrive, 030 accelerator with at least 8MB extra RAM, but you will have maintenance issues like recap and so, but a good mid range setup. You will have a chance to run a nicer OS (Workbench) setup with higher resolution and more colors, games (via WHDLoad) will load and run faster and more conveniently, nicer demos etc..
Perhaps watch some youtube videos first of these two setups to see what to expect. For more hands on experience, configure and run these two setups in WinUAE.