When most people decide to build an electric bike they usually don't think much about the type of frame they use. Often they use any old bike they have lying around or go buy a cheap bike from Walmart. This is fine if you are building a low power ebike but if you want something faster and more powerful than the average electric bicycle you need to choose your bicycle frame carefully.
The head angle or angle at which the fork comes out of the frame affects how the bike handles. The steeper the head angle (closer to 90 degrees) the twitchier or more unstable the bike will be at high speed. A slacker head angle (closer to 0 degrees) will be more stable at high speed but lose some stability making slow tight turns. A slack head angle also lengthens the wheelbase of the bike, which further improves high-speed handling. A freeride or downhill type mountain bike frame will have a head angle anywhere from 64 to 69 degrees depending on the intended use of the bike so these make excellent platforms for a high speed ebike. Bicycle manufacturers will list this info on their website under frame geometry (aim for 66 to 68 degrees) so you can find out the head angle of any bike frame by checking online.
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.