4.8 to 6.0 volts, either NiCAD, or NiMh are just fine. 700 Mah is fine for a plane with four regular servos, this will give you approximately one hours flying time. If using more then 4 servos (most designs now use 5) I would go with higher capacity. Higher capacity and NiMh will also give longer flying times. People do use Li-POLY or Li- ION cells in the big gassers, however, I think they run them through power panels first.