Depends why you modify.
I modify car's because I enjoy it. I like doing it, and seeing improvement in performance and looks bring enjoyment.
It is expensive, it would probably be better to buy a better base car, but then it would have the feeling of "I built that".
Why don't you just buy an already -well- modified car. It is much better value for money than doing it all yourself.
As for coil-over's not suitable for road use. Why don't you buy coil-overs that are decent rather than the highly marketted one's? I bought cheap one's for my 200sx S14a along with locking collars and it was the best 'mod' I ever made. Handling was incredible (for a RWD). It was bumpy but then I didn't mind it, however the wife did. So whether something is 'suitable' is down to personal preference.
If your just after a fast car, unless you are really into Fiesta's/Ford's just try something else that gets good reviews and is faster to start with. Obviously if you start with a Forced Induction car, making it faster will be a doddle.
Have you considered getting a BMW's diesel turbo with a remap. Been very impressed recently by them, but obviously handling won't be the same as the Fiesta.
Whatever you get, if it's fast and handles well, you'll have to compromise comfort at some point.
Personally, unless your prepared to spend big money getting a charger kit done properly on your car, I wouldn't bother. The last thing you want is a blown engine because you cheap'd out.
Good luck with whatever you get
