In terms of air drag, adding an electric motor+batteries to a bike makes almost zero diffrerence. Lower center of gravity will not reduce air drag. High center of gravity will make a bike handle somewhat differently. If your bike has high center of gravity and all of the extra weight is on the rear, it is likely you will not like the handling (although I personally don't mind having a lot of weight up high in the rear).
If you want zero drag, get a "geared hubmotor" or a motor system that is not built in to the hub (the walmart/schwinn E-zip and the cyclone-tw.com kits, for example)... because all of these have a freewheel so that the motor doesn't have to turn while the bike is moving.
I put a lightweight motor+battery on my bike for a total e-bike weight of 45 or 50 pounds and the bike is very rideable without the electric power. It is noticeably harder uphill due to the extra weight though. That will be even more true if you go with a 75+ pound e-bike (Lead Acid batteries or big hubmotors will make your bike heavy.)