The Apple III was intentionally designed to be compatible with the Apple II
Plus, albeit with some artificial limitations. There are a few hard to
come by cards that can improve it's compatibility, but viewing it as a
II, it's a 48K system.
The III itself is an interesting, charming, frustrating and at times
maddening machine. If you like to tinker, the III will provide plenty of
challenges -- especially if you get an older machine. The overall lack
of III specific software to explore may be discouraging but there are titles
that are very worthy, mostly business-oriented apps.
I used to have quite an assortment of III gear but ended up giving it
all away to Tony Diaz. Sometimes I regret it, because I didn't "kick the
tires" enough to really appreciate the platform. I think that despite
it's glitches, it's a great machine to explore and you can get a sense
of where Apple was trying to go in the early 80's. You can also see where
the early IIe earned it's dual pedigree, from both the II Plus and III.