You'd be much better off getting your own battery and charger. The batteries they ship with guns are usually crappy and the chargers they ship with it tend to put them on fire.
Get a B6 charger from Hobby King along with some cheap Lipos, you won't regret it.
http://www.hobbyking.com/hobbyking/s..._GENUINE_.html
For an MP5 a battery like this would be great:
http://www.hobbyking.com/hobbyking/s...0lipo%20nano-t
If you go for a full stock MP5 you could go for something bigger.