Speaking from four years of 16V/Willy ownership, there arent really any weak links specific to the car apart from rear arch rust, wheel bearings, engine mounts and high labour costs arising from the fact that it has a large engine in a small bay.
Other than those, IMO the things that go are usual for an older car. Thing is, a 16V is very cheap to buy for the speed/handling you get - but costs more than average to repair. There are loads of things to watch out for like worn suspension/steering components, cambelt changes and starting/charging systems, although I think youd be watching these areas with any car of the age. To have your 16V handle as the Renault God intended, youd probably want to change most of the things that wear out in the suspension (bushes, links etc).
Some people have trouble with gearboxes, but (touch wood) Ive never had that in over 100,000 miles of combined 16V/Willy use. If you buy wisely, a lot of the key components will prove extremely durable. If you buy a poor car, it could take a LOT of cash to get on top of all the problems (as happened to me with my first 16V).
£2,500 will get you a very tidy Valver these days. Its worth taking a forum member with you to see any cars as you can easily get carried away when you first see a 16V for sale.
Id personally say that at least half the Valvers youll see for sale are right nails but that there are some very cherished examples - watch out for club cars. If youre willing to take your time and do some research, youll have no trouble finding a good example.
BTW - if you enjoyed your 106 GTi, youll love the Valver. Its not really any quicker until 3rd gear and its not quite so keen around mini-roundabouts - but its much more stable and the damping control is way superior IMO. The Valver makes an excellent B-road car as its very controllable on the limit.