IMPO start with the basics, itg replacement panel filter, exhaust, de-cat and on the Clio's match the inlets and get an rs tuner map, and get a larger air feed
You'll probably spend nomore than 700 quid and gain around 10-15bhp
If i were going to charge my car I'd want all the above done anyway to improve the flow of air etc in and out of the motor, whether it be turbo or super charged
as for prices
ITG for ph1 airbox is £33 from merlin motorsport
Good quality exhaust and de-cat £450 new, maybe get the full lot 300 second hand
matched inlets £150 on an exchange basis
RS tuner 150 including unlocking
I'd prefer a good quality second hand exhaust with all baffles/welds intact
there you go, done for 6-700 quid, not a lot considering you have to do the foundations for any other car aswell
If you're truely short of cash, start by taking out the spare wheel/jack and any other junk you may have in the car, less weight, better power to weight, free speed, less weight to slow down so should also stop better