First, no SVT Focus came stock with a supercharger, but you may be able to find on with an aftermarket system on already, but those have probably had a hard life. I would stay away from a Saleen since they have N20 but if its hooked up the warrenty is voided and there are no supporting mods, so its basically a stock 05 focus with nitrous, stickers, and an awful bodykit. If you want to go newer buy a Focus ST, its about 20hp down on the SVT but has the same suspension and more displacement(2.3L compared to 2.0), but only available as a sedan.
All supercharger kits have been discontinued due to lack of demand but there are a few places for a turbo:
xiosmotorworks.com
c-f-m.com (great place for all aftermarket SVT parts)
Here are some pretty good articles with facts and figures about the SVTF:
http://www.caranddriver.com/reviews/..._test/(page)/1
http://www.caranddriver.com/reviews/...parison_tes t
Forum with tons of info:
http://www.focusfanatics.com/
Common problems:
DSI clip breaking (only on 2002s though)
Clutch (stock ones are hard to find so most people get an aftermarket setup, which also means getting a new flwyheel)
Endlinks breaking
There is a YR40 reflash given out that fixes the "surge" when driving
Basically get a 2003 or 2004 since all the issues are mostly ironed out. I have an EAP 2004 ZX3 and I love it. Not that quick in a straightline but in the corners it embarasses quite a few more powerful and expensive cars.
If you want a balls out straightline import though I would have to choose an SRT-4,but if you want a well balanced car that has a great interior, cornering, and some pep the SVTF is a great choice.