Your best bet would be to wait a while and then buy used. The way the technology is moving ahead by leaps and bounds, using Zero as an example, a year old bike would likely meet your price point. But range will still be a problem. The 2011 Zero has been tested to run just over 20 miles at 65 mph. You might get 40 to 50 miles out of the bike if you kept it down to 25-35 mph on level ground. However, the 2012 Zero claims over a 100 mile range and a top speed of over 80 mph (but not at the same time). Unfortunately, its price has increased to $14,000 USD. But that does nothing to help out resale values of the older models and that is where you need to look - if there is anything to actually see.
Frankly, I doubt that you could build your own electric motorcycle with your performance requirements within your budget at this time. Just the required batteries would set you back most of that and the motor and electronics would take up the rest (as least). The donor bike and all of your labor would have to be on you.