I'd advise getting a PS2 if you want to play the majority of FF games. They're very cheap these days ($50-$100, I think?), and you'll be able to play most of the main story games.

Sure, you could use emulators, but I don't know if there are any PS1 and PS2 emulators that work on the Mac - you may need to install Windows to run them.

Oh, and FFXII: Revenant Wings for the DS is supposed to be very good.