The Amiga Scene: I'd like to come back but don't know how!

[ajk]

While A1200's are quite cheap,  they are pretty much as modern as Classic Amigas go, and offer reasonable expandability. Expanding one with all the available gear will be hideously expensive though, but for nostalgic purposes that isn't necessary. A hard drive, some extra memory and maybe an accelerator and a scandoubler and it's a fine system.

For an up-to-date system there are newer options, which have already been mentioned in this thread.

[ajk]

Aside from OS4.0, which is the best all-around 3.x series? I've seen 3.0, 3.1, 3.5, and 3.9 come up a lot.

3.5 and 3.9 both come with 3.1, so either one is quite ok. You will need to update a basic A1200 a little to run 3.5 or 3.9 well (extra mem, hard drive and such), but 3.1 is ok with a basic system and is still reasonably well supported. Anything older than that isn't really useful with an A1200 anyway, since it has either 3.0 or 3.1 kickstart ROM's inside it (update to 3.1 if you happen to get an old model, the new OS's require it). There is really no reason to use 3.0.

There is of course the thing that 3.5 and 3.9 arrive on a CD, 3.1 was still floppy based. A CD drive will be quite useful though, for transferring stuff from a PC in the initial install phases (a lot of stuff can and need to be downloaded from the net). Naturally a hard drive is a practical must-have for anything else than nostalgic gaming.

For really old games it's better to get an A500, but that's a different discussion.