2 very long posts here on SMJ about what makes a good street tard: http://www.supermotojunkie.com/forumdisplay.php?46-Street-Tarders
I've got an 06 WR450F supermoto street-legal tard and it is just plain badass. Tend to ride my stock DRZ SM more though because it seems more comfortable and quiet - better fuel economy as well.
Not sure exactly, but it can't be that great. The WR has the big carb, pipe, and shorter gearing. My DRZ has a 16T c/s sprocket on it and now gets better than 60 mpg even riding it hard.
If you're gonna pick a dirt bike to convert I'd say the WR is the one to use. I've had a couple of them, one I rode on the street for about two years nearly daily. Rock solid bad ass bikes.
I get around 80 miles out of the stock tank with my 05. Do a search here, I know there's been a few threads about converting 450's.
Yeah, inconvenient but worth it. At the time, my commute was 40ish miles 1 way, so it wasn't too bad filling up once a day. I got a Valdeze (sp?) size tank with the bike, but it ruined the way the bike felt with it full.