I would check out the Magic Pie 5 by Goldenmotor or a similar brushless hub motor. It retails for a little more than $400 without a battery. It will go to 30mph quite quickly, amazing torque. You can find a good bike on Craigslist everywhere for around $200. Battery packs can be found at various places in different voltage for around $300 and up. I think you could make a great bike with around $1500. My recent build is on an older Specialized FSR, with a Magic Pie 5. I paid around $900 for battery pack and motor, and $500 for bike. Start with a good solid, comfortable bike.
Disclaimer : I only have experience with two ebike motors, Magic Pie 5, and Heinzman 500w (EVG E-Bike); and I distribute for Golden Motors; however, I am only suggesting the Magic Pie based on my experience, which led me to becoming a distributor and e-bike enthusiast.