What internals to buy?
Most of the time, Opening an AEG is a bad idea, unless something break, or unless you really know what your doing.
Your probably more likely to break something, and parts are expensive...
Also, ICS have a number of parts in their mechboxs that are not compatible with most mainsteam parts. Keep that in mind, but G&G is definitely a more upgrade-friendly gun.
It's better to light a candle, than to curse the darkness