Dan is spot on.
Most imports have been repaired in some way. When checking lift the carpet in the boot and also lift the rubber seal on the doors. Look for trapped dirt, as Dan said, under the paint. Also check for sort of wavy lines running from the roof down to the boot. Check of weld repairs in the boot, around the door sills and the engine bay. Particularly look for welds in the engine bay. They may look like they should be there so take pics if in doubt and compare them to cars here. Also look for uneven gaps in the panels between the bumpers and the fenders (front and rear).
Low mileage imports are the ones to be most cautious around. The clocks may of been tampered (happens a lot) with to entice someone to part with their cash even though they know about the repairs.
Best advice is to take your time. I'm an impulse shopper myself but when you're about to part with 20k you've got to be sure it's worth it. There's a lot of Z's for sale out there so there are plenty of good ones.
Guy