I'm going to disagree, and offer some further brain-food. First- you certainly CAN use the sprocket as a brake rotor. This is a factory setup done on some riding mowers that use a differential equipped rear axle with a separate transmission. The rider that I moleted for parts for my kids' go kart used this arrangement. While I did not elect to use the mechanical caliper that came off it, I DID use a variation of the same concept with a hydraulic motorcycle caliper. The axle sprocket is bolted to one side of the diff housing. I bolted a SECOND sprocket to the opposite side of the housing to serve as the brake rotor. Thread here-
http://www.diygokarts.com/vb/showthread.php?t=12904
Please note that braking the diff housing does create one slightly odd characteristic- Due to the very nature of the differential, slamming on the brakes at high-speed in very loose traction conditions (like gravel) can actually cause the wheel with the lesser amount of traction to spin backwards briefly. Yeah- you did rear that right. Wanna know why? Jack and differential-equipped axle off the ground, and hold the input from turning. Now spin one wheel by hand. You'll see the opposing wheel spin reverse to what you're doing.
Obviously, this does have the potential to adversely affect braking performance under the wrong conditions, and you truly are better to brake each axle seperately. So far, my kids haven't had any serious issues with it.
For rims, go get some lawn tractor rears. They are typically 3/4" keyed or cross-drilled bore, 8" diameter.