As Trewy said rust will be the biggest issue, check front/rear wheel arch's, sills, top of arch where mid door's close (5-door model), rear cross member and boot floor. Take a screw driver with you and get the carpet up to check the boot floor, if owner objects walk away, it's only 50 notes for a new boot floor but can be a git of a job to do.
Seat belt mounting points.
Expect some of the electrics to be playing up, usually the window switch's.
Check all the correct size fuses are in place
Engine's normally good for 250K + if cared for, but check all the levels & for leaks
Power steering pipes can rot & leak
Gearbox, usually 2nd is a bit notchy, 4th & reverse can be a pain to engage on some.
Most of all take it for a decent run, by the time I went to buy our 2nd (current) Disco I'd learnt so much from the 1st one and it's issue's .... this time round I got under & into every bit I thought might be an issue (thanks to advice from LZ'rs). I viewed some that I would not even drive in a field, took me 9 months to find our's a really tidy example with full service history, but even she had a few minor issue's.
Take your time, there's plenty out there for really silly money at the moment.