Hmmm...this is the age old question for Renaults. By and large, the Clio (all since 1991) are pretty tough. The 16v/Williams are a different case - though arent "unreliable" or poorly build - more on this in a moment.
The 1.4 Energy engine is very straightforward, is dead reliable and is tough - provided of course its serviced regularly. Ive actually never heard of a 1.4 that isnt modified or ill-cared for causing any trouble. The RT and other "normal" Clios will generally give very little trouble in terms of mechanical parts and they cope well with the power. The key thing is to keep on top of anything that is prone to deteriorating (like cv joint rubbers etc). Also, many RTs found homes with middle class women who have kept their cars well becuase of they have plenty of cash and concern!
Best not to buy a moded one with high miles I reckon. Lowering in particular is not kind to all the suspension and steering parts - and gradually youll find that you start to get more of the problems associated with the 16v/Willy...
...Which brings us neatly back to those cars. Wonderful to drive, fantasticly fast and big arches/bumpers/bonnet bulge mean theyre beautiful to look at. But a big engine in a small car takes its toll on all the other parts. For example, the chassis (despite being very stiff) still flexes enough to let water in through the sunroof, the engine mounts in particular cant take the power for much more than 60k miles and the whole steering/suspension moving parts assembly will keep you busy with problems beyond about 70k too.
Basically, Ive had three Clios and none have ever broken down. But the 1.2 caused problems after many miles combined with lowering, the RSi just got tired with all the miles - and my 16v doesnt like all the power the engine gives it!!!