After reading this forum for quite a while now I can't really think of any glaring faults to look for. It's keithblade said, it's a pretty rugged bike.

I guess look for the usual stuff like worn sprockets/chain, dented rims, scratches, a service manual with stamps in it - that sort of stuff.

The regulator rectifier is the only thing that sticks out on the older bikes, but even that is probably sorted by now

Good luck