I havent seen many moddified BMW's the most I saw was when I was at uni in Birmingham and the 'rude boys' all drove cheap 3-series BMW's and had bought/stolen the 'M' badges and alloys and stuck them on! Seriously, there are 1000's of BMW M3's in Birmingham!!
The most common moddified cars I see are 'cheap' hatchbacks, Vaxuhall Nova's, Clio's, Fiesta's etc etc.
I had a really nice Rover Metro Sport as my first car (yes I can hear you all laughing!

), it had a K-series engine so for what it was it was quick, I could put it into a Mario Kart powerslide - if the conditions permitted - shame I lost compression on one of the cyclinders...basicly the engine went 'pop' whilst I was on the motorway

I loved that car, despite it not being moddified that much.