Take someone with ya.. very valuable really.
When Mat and me went to look at the loads we did, we pretty much found problems with them all.. apart from the odd one or two. Two people, between ya, spot a lot more and can tell the good ones from the bad.
There are a million things to look for, but a lot are common sense.
Clutch, Steering, Paint (depends how picky you are), interior, engine bay, oil, heaters, oil pressure, gearbox etc..
Oh, be really picky about paint.. (i.e. No spray work) and get a true minter - might take a year or two tho.. heheheh. Again, depends how picky you are as to how long you wait/how many you look at.