I would strongly suggest that you checkout the Marin Muirwoods 29er bike. It's part Urban Commuter and part mountain bike. You can get it now on sale with Amazon for only $450.

Buying used can really be a crap shoot, unless you really know what you're doing...

Always buy new, if you can!

It's made of chromoly steel and has decent components.

5' 8" = 17in