I've recently just bought my 172 CUP. I've also owned a Fiesta ST previously.
tbh, I had a lot of niggly little problems with the CUP it when I bought it... thinking you are going to buy a CUP for around £3k is ok, but one thing to remember is its going to be a 7 year or so old car, so make sure you try and get one with LOTS of history.
I paid £3390 from a trader, and he had the cambelt done and a 12 month MOT on it. After getting it, it was going into limp mode and idling bad and stalling. Turned out my throttle potentiometer and throttle body were knackered - so both had to be changed. Luckily buying via a trader I had 3 months warrenty so negotiated with him to pay for all the parts - my mates father owns a garage so I had all of the work done there. Now, its running perfectly, I also had 2 new tyres and all the tracking done etc.
The car itself is great, I easily get 40+ mpg on a motorway run if I keep it around 70mph. If you put your foot down it pulls like a train... I've recently owned a Fiesta ST (modified with manifold, sports cat, cat back, induction kit, eibachs, dreamscience remap) then I moved onto a Focus ST - I can honestly say the Clio is every bit as fun as the Focus (though I miss that 5 pot growl.. more than made up for with the Cups' handling....) - the Fiesta... I enjoyed owning it, but its a warm hatch at best - to get any sort of excitement out of those you have to at least do the mods I did, by which point you'll go deaf from the exhaust on motorway trips...
I think if your motivation is entirely financial, I don't know if I would bother. If you start to put things like driving experience/fun factor into the mix I'd say the CUP is leaps and bounds better than the Fiesta ST, but you are buying an older car, you won't know how its been treated, and potential costs might mean you are no better off than you were with your Fiesta ST.
A buying guide for the regular 172 can be found here:
http://www.strikeengine.com/renault_clio/buying_guide.html
covers the main things to look out for. I did see some complete sheds, vague brakes, 17" alloys etc. Be prepared to travel... lol.