An Emax is just an X-valve with Emax engraved in it instead of an X. Its a bit special, just for that reason. It does have an level 10 bolt. You will need an X-valve parts kit for orings.
The battery is personal preference. If they are the same price, I would get the largest capacity one. If you don't have a charger, you can get a new one, or you can get any aftermarket charger that is capable of charging a NiMh 14S (16.8V) battery pack.
If you can get rid of the paint coating, you'll probably be better off. Paint has a tendency to throw the tolerances off. Paint also doesn't hold up to paintball impacts as well.
Except for the Automag in front, its usually the man behind the equipment that counts.