I'd suggest trying a kids full suspension steel frame Mountain bike and convert it to a BMX style. That's what I did. They are very addictive and always beg you to add more power. A high speed hub in a 20" wheel setup fed the proper amps is soooo much fun, but be careful of wheelies when changing geometry. I had one combination that was completely unridable at one point due to it looping me off the back several times by accident because I messed up the geometry so bad.
Started with this, very similar to a cheap Kent steel frame bike I picked up to make another one. Brand new entire bike is $150, got both of mine used for $20 and $40 (second one is being modded now).
Changed a lot of stuff including the rear shock to get the ground clearance I wanted, powder coating, BMX handlebars, better linear pull rim brakes (good upto about 40 mph max, you really need good big disc brakes after that if you want some resemblance of safety and the ability to panic stop quickly and controllably).
Now it's 8" extended swing arm, 26" front fork, 203mm hydrualic disc brakes and way fast. Rode it to work today and find it more comfortable than my full size mountain bike. I think my full size just needs higher handle bars to solve the problem of comfort.
Greyborg Warp Frame with Greyborg Cromotor aka "Monster Bike
", 24 FET IRFB4115 EB324 Top Speed 42MPH (75V)
Greyborg Hub Motor, ZombieSS 24FET/36FET IRFB4115, Customized full suspension kids MTB. Top Speed 61 MPH (125V)
9C 8x8, 24S2P LiPo, Lyen 12 FET, Diamondback Recoil Comp. Top Speed 42 MPH (100V)Greyborg USA/Canada Dealer, Frame kits + motor in stock now http://www.greyborgusa.com