From what I've been able to work out, importing a non-stock vehicle to Europe is fine as long as it is still classed as road-worthy in the EU - i.e. check weight, emission etc restrictions with your local test-center.
The problem is that if it's not stock it will have to go through a full test as they wont have an already approved vehicle to test it against.
So basically you'll have to sit a full Single Vehicle Approval (SVA) test yourself. This is where they check the car from top to bottom and comments on anything you'll have to change to get it approved. Once approved you can then get it registered, insured and finally enjoy the ride
I'm guessing this is pretty much the same in all of Europe as it's set by EU guidelines - and although some states (Sweden in particular) have managed to opt out of various clauses (booze, tobacco etc) the vehicle tests should be the same all over the place for safety reasons.
Of course, if you want pretty much a stock vehicle, but stock, get it approved using the short test and add your supercharger etc afterwards - at which time it'll still need to go through 'bilprovningen' but should pass rather easily as they then only check your modifications. Also, make sure you'll get insured with mods - I know the insurance companies are quite ok in Sweden, but still something to bear in mind before buying/modding.
Hope that helps somewhat.