Sills! Ask him if he's ever had the covers off, if so are there any photo's, if not ask him to take them off and send you pics and if not take them off yourself. If he declines walk away. may sound silly to some people but i looked at a car identical to that, in the pics it looked even cleaner than that on. guess what? i bought a nail! Check the guttering. if the rubber is lifting away anywhere or there's bubbling, walk away. If he doesn't like the idea of you removing sill covers ask to remove the rear door cards and get a good look in the crevis (take a torch) is there any sign of rust? if yes, walk away or be prepared for possible heaftey welding costs. There are still clean mk2's available for under a grand but generally i try to look for pre 90 as they don't have sill covers and often no arch trim so they cant hide the nasties as well. You can always add big bumpers and sill covers later, after all that's the fun bit. But i assure you your dream will be shattered if you play with it, love it and tinker with it until you get to mot and presented with 6 pages of fail.
Good luck dude, if its sound then it's a bargain!