Craigslist is a great place to get deals on used mountain bikes that are perfect
for your electric bike conversion and will save you money that can be better spent on a quality battery pack. You can purchase a full suspension frame which will give you a nice smooth ride if you don't need a long range bike ( the rear suspension takes up space where you could add more batteries). If you want maximum range from your bike a large size hardtail frame (no rear suspension) is your best choice as there is plenty of room inside the frame traingle to mount a large capacity battery pack.