There's quite a lot to look out for, I'd suggest taking someone who knows Landys with you if you go see one that takes your fancy. Here are a few pointers for you though,
Disco - Bits that rust away....Chassis, Sills, boot floor, wheel arches (below rear door striker plate in particular) inner wings, door pillars, sunroofs, (prone to leaking), electrical bits, just make sure everything works ok,
90/110- Bits that rust away....Chassis, outriggers, bulkhead, door pillars, footwells, rear crossmember,
Oily bits....These are pretty much the same for both,
Engine, is it smokey, blue/black/white? Start ok? Any signs of leaks?
Gearbox/transfer box, run quietly, change gear easily, any whines or knocks?
What do the fluids look like, new or dirty and old?
This isn't exhaustive by any means, there are plenty people on here that will help with advice and pointers, I've owned a Series 3, 200Tdi 90, 300 Tdi Disco Commercial and XS, Freelander, and currently run a TD5 Disco 2, pretty much all of the issues I've had have been wear and tear stuff, pads, discs, bearings, etc.
Good luck and hope you find one you like!