Lipo comes in voltages of 7.4 and 11.1. there cells are 3.7 volts, you will also need a lipo charger with balancer. lipo offers different voltages and mah configurations and c ratings a mah of 1100 to 1200 with a c rating of 20 to 25 should be more than enough and still be able to put the battery in your front end or buffer tube. the larger your mah the larger the battery gets therfore the necessity to add a peq box or mods to your rear stock.
just some things to keep in mind when getting a lipo"and you will need to excersize trigger control to save the internals of you gearbox from wearing out prematurely.most of the time the piston gets stripped and or the trigger contacts get burnt from arcing "adding a mosfet eliminates the trigger issues.