you can't do any worse than picking up a secondhand A500 off of ebay.
if you want the full floppy loading expirence as well then this is probably your best bet.
but as ever, depends what you want....
The A500:
very very compatable, but hard disk expansions are becomming rarer, so chances are, you will be limited to floppy disks only.
The A500 Plus:
comes with a more uptodate chipset, but looses some compatability in the process - not enough to really worry about. Same expansion options, but comes with more chipram (graphics memory) so comes in handy if you do decide to put (and can find) a hard disk on it to run WHDLoad modified games from there.
The A600:
has all the features of the A500Plus, including a space for a standard laptop hard disk, in a very small case, but without the expandability.
problem, it cannot be expanded with memory beyond 2Mb, or faster CPU's without alot of effort. people buy A600's as an exersize in "how much can i expand the non-expandable" :-D but otherwise, quite a cute little machine.
The A1200:
Faster CPU, and best chipset, able to find ram cards, and processor accelerators much easier than for the A500, able to take standard laptop hard disks straight away. max amount of graphics ram, so no problems. can run a "degrader" to run games that have problems otherwise. can run WHDLoad very well. for max game compatability with an A1200, i wouldn't advise going past an 030 CPU.
old old games like Populous run their game timing directly off the CPU clock, so the faster the CPU, the faster the game!
If i was going for a bit of a knock about with a couple of games, and then put it away again, I'd go for the A500.
If i was going to have a bit of a play and leave the machine out as a bit of a hobbie retro gamer, i'd go for the A1200.
Bear in mind, a hard disk on an amiga will drasticly change the way you use the machine for the better. has to be one of the best things you can add if you are going to get into using one on a regular basis. :-)