some of the higher end hub motors have threads for a disc brake spider to thread on, and the reason I recommended a hub motor is that they are the best option, the motors are much more efficient, they don't have the wear of a friction roller, chain, belt, or gear box, hub motors normally give you better range and higher speed for a given battery size, and fewer batteries save both weight and money.
Another rout you can take is basically building a Bob trailer (single wheels bicycle trailer) that has the battery and motor, and the only part you would then have on your regular bike would be the throttle, this is going to make the whole rig a bit heavier, but it's going to open up your options for motors, and batteries and keep your center of gravity lower.
I like to build things my self as well, and have built a number of electric bikes in the last 10 or more years, I have an old ZAP electric bike kit, a small pile of other 24v and 36v 1/4hp-1/3hp electric motors, and a
Crystalyte Phoenix hub motor, because I want a truble free electric bike, and the 25+mph top speed is really nice, but of course I was willing to spend more then your entire budget just on the electric motor, then do the same thing again with a Lithium battery pack, and am building it on to it's very own frame, keeping these heavy components off my light weight bicycle.
so tasdrouille, what are your ideas on how to build an electric bike?