The choice between a 172 and a 182 is entirely based on budget now. Ignoring the phase 1 172, the 172 and 182 can be considered the same car, on a sliding scale based on how much money you have. Its just at some point around 2004 you will go from 172 to 182.
However, for your budget, your safely into 182 territory, so don't worry about 172's. What you need to look for is key things in terms of the condition of the car. Dont worry so much about mileage. Major items are service history being up to date, and the belts services being done. Aux belts should be done at 36k and Cam belts at 72k. Avoid Clios near these mileages without the belts done, as the 72k service from Renault will set you back £800.
The next thing to look out for is spec. The 172 was simple, there was the 172, with all the toys, and the 172 Cup, without the toys but lighter. The 182 gets more complicated. There are still the stock 182 and a 182 Cup, but there are also 2 Cup Packs, one for Styling and one for Handling. The Styling pack puts the larger Cup spoiler and front splitter and Anthracyte alloys of the Cup on the stock 182, and the Handling pack puts the Cup suspension on it. The ideal car therefore is a 182 with both Cup packs, often referred to as a Full Fat, or FF. This has all the toys of the standard car (leather, climate, cruise, auto lights, auto wipers, etc) but also has the look and handling of the Cup. This is what you should go for.
So then its just finding one. I think for £4k you would have a fair few early 182 FF's to choose from. Join Cliosport.net for advice too. Being just a forum member (as opposed to a paid club member) still gets you access to the Cars for Sale forum, so worth a look.
It is also worth mocking up some insurance quotes to get an idea. RS Clios carry high premiums so expect insurance to be quite a lot.
As for things breaking... there small French hatchbacks, so its par for the course really. Most things other than the belt services aren't to bad though. Regular failures are rear shocks (£100 for the pair), exhaust mounts (£25 each), and other assorted things. Electrics are typically French too.
Oh, and definitely get the 182 and not the Lupo!