Since a supermoto is just a dirt bike with 17" rims maintenance is gonna be high. Oil change every 3 rides or so, valve adjustments every 15 hours, top end rebuild every 90-125 hours. If you like wrenching and upkeep it aint that big of a deal.
Sumo's are a freakin blast to ride! Wheelies, stoopies, stairs, sidewalks, curbs, loading docks, industrial parks, railroad humps, short cuts, cheap to race, tight canyon roads, you get the idea! They do have a few drawbacks though, maintenance, 2x4 seat, not highway friendly, small gas tank and they will cause you to do stupid things!
There ain't nothing more satisfying than coming up to a tight corner pretty hot, on the brakes hard, slide up on the tank, drop a couple gears, slip the clutch, and "backing it in" the corner!
With you living in California ur limited to what you can get easily. Suzuki makes a drz400sm and yamaha sells a wr250x, both are factory street legal Sumo's. Both are a bit under powered and the drz is a bit heavy. But both do have a lot of aftermarket support. And they both have easy maintenance schedules. KTM makes a couple big bore Sumo's too, they arnt as nimble as the smaller ones are though. You can build your own if you can get a street legal dirt bike. These have more power but require the heavy maintenance. KTM sells street legal bikes since 07, the exc.
<-- good info
<--- place to shop for parts