I would get the ICS L85A2, its better. G&Gs ETUs are very hit and miss, plus I've heard the blowback can lead to premature death of some of the internals.
L85s take standard M4 AEG STANAG style magazines, as for the battery, its stored in the handguard so this is dependent on whether you want to swap out to the DD Rail in the future. As the plastic handguard will have more space for a battery.