I do not recommend a 2-stroke motocrosser for street duty, though I have one myself. The only reason mine is street-legal is so I can stay familiar with the bike by riding it around...it is my race bike.
For a mainly street-ridden motard, the Honda XR650L is a good choice. White Brothers has a bunch of parts for it, it's already street-legal, and is probably the easiest choice. If you can manage to street-legalise a dirtbike, the XR400R is another good choice, as is the XR650R (these last 2 are kickstart-only though).