Desirable bike characteristics for electrification:
1) Big steel frame. Aluminium is OK, but steel is stronger and more durable.
2) You need "flat dropouts" if you plan to go with a rear hubmotor.
3) Big, flat handlebars so you can mount stuff on 'em like a throttle and/or the control head for the electric motor.
4) A relatively big main triangle to hold the battery. You don't want to put the battery on a rear rack or front rack, you want it in the central triangle as low as it can go.
5) Good lighting: most bicycle lights are insufficient for ebikes because they don't illuminate enough distance in front of you. I'd say any headlight you choose should be 250 lumens at the minimum, and 500-800 would be satisfactory. You want the biggest, brightest taillight you can get, too. If you are handy with 12 volt electrical systems, it's not beyond possibility that you could make your own headlight and taillight systems from materials you could source at WalMart or an auto parts store.
Suggested Bikes:
Electra Townie 7, "Balloon 7D" or "Original 21D" would all make nice ebikes:
http://www.electrabike.com/bikes/townieElectra actually makes their own ebike, if you want a complete one:
http://www.electrabike.com/bikes/townie-goIf you prefer a step-through frame, this is a decent bike to convert:
http://www.performancebike.com/bikes/Product_10052_10551_1186084_-1___400863Got those nice flat dropouts on the rear wheel, usable suspension seatpost and front forks. Nice bike.
Pretty decent pre-built "metro" electric bike:
http://www.performancebike.com/bikes/Product_10052_10551_1177386_-1___000000