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.
