I know I'll regret saying this, but honestly I think you would be better off buying a factory supermoto than doing a conversion on something not meant to be a street bike. But of course a lot of your decision needs to be based on your riding needs, how much maintanance you like to do, and what sort of longevity you expect from the bike. Based on experience I've learned from several of my friends that ride converts daily, I wouldn't want the hassles.
Now with that said, I got a chance to ride a KTM 690SM today. Good lordy thats about as much fun as one human being should be allowed to have while still sober and clothed my friends.... :arsenal