Just echoing what has already been said really. Rust on the arches, sills and boot inner. Tend to eat wheel bearings as do most mk1's - especially if theyve been lowered. Look for a good oil pressure - the dial on the dash should read around 3 on a cold idle, just above 0 on a warm idle, and around 3 at 3000rpm when warm. Expect it to go up to 6 if revved when cold.
Power steering is a common problem, if its noisy check the fluid levels - yellow rectangular box on drivers side of engine bay, if its full then your likely looking at a new p/s pump which can be expensive and is difficult to fit - same with gearboxes - if there is lots of nasty noises or it dislikes certain gears your looking at big bills.
Cambelts need to be changed every 70k but its preferable if its been done sooner than that, these are costly to replace so try find one thats had a new one in recently, if you buy one that hasnt been changed for the last 70k expect a sever credit card assault when it finally goes twang.
Same with clutches really, absolute b*****d to replace and as such its an engine out job which again means ££££'s so find one thats had a clutch recently or factor in the big cost of a new one in the asking price.
When looking for one, make sure it is being started from cold, look for any blue smoke from the exhaust, if there is then walk away. Dont be worried too much about slight ticking from the engine, but if there is a knocking lower down then leave as this could well be the death rattle.
On the inside as has been mentioned leaking sunroofs are a very common problem, check on the outside first to see if an attempt to seal it up has been made before - if it has then dont be alarmed, this is usually a good thing as if done properly it means theres less chance of you getting wet. If water has knackered the electronics in the central locking mechanism which sits just below the sunroof then it'll be yet another expensive trip to renault.
If the dash is dim then the bulbs need replacing, this isnt that difficult or expensive to do but obviously it is a dash out job. If the speedo itself doesnt work then the speedo cable could be knackered or its a problem at the gearbox end which could be a pain to sort. If the rev counter is adrift then usually its the unit itself thaty needs changing. This isnt too hard to do once you get the dash out.
Another problem sometimes experienced is the lack of heater blower. The heater will get warm but not blow out air, this is usually a fault with the multi plug that connects to the blower unit under the scuttle panel, or perhaps the blower unit itself.
Finally, dont take sweets from strangers or accept any offers to see 'puppies'
Hope this helps
Tom