That's alot of questions!
If you're looking for an economical and fast car then the ST170 is neither.
Turbo charging is going to cost in excess of £5k, add the cost of the car in and you may as well have bought an RS...oddly they can be more economical than the ST170 if driven sensibly.
Personally I'd go for a good, late (2004) 2.0 ESP as I own one and can't fault it, 40+mpg on the motorway and faultless reliability. A TDCi sport is the sensible, economical yet sporty option or an RS if you can afford it and don't mind putting those kind of miles on a future classic, you can pick them up very cheaply these days.