Both times I was getting into the Amiga, I found the A500 to be quite a serviceable machine to get acquainted on. They're inexpensive and readily available, and have a pretty reasonable variety of upgrades available for surprisingly little.
If you want to get a feel for Workbench and Workbench-based software (as opposed to games and the like,) you'll want to put a 3.1 ROM in and use Workbench 3.1, and likely get extra RAM and a hard drive, but even a WB1.3 A500 isn't a bad way to get acquainted.
I would say similar really (ie A500 for classic Amiga gaming) but if you also want to try out Microsoft Office style serious applications on WB3.0 WB3.1 etc then probably cheaper to get an Amiga A1200 and a cheap laptop 4gb IDE hard drive rather than try and make an A500 into that sort of spec with extra (expensive) bits and bobs.
