Best. Line. Ever.
As others have said, try 0.25g. All plastic BB pellets will get blown off course, it goes with the territory but unless you have a spectacularly bad gun you should be able to use 0.25s just fine which will be better.
As for guns - anyhting with a V2 or V3 gearbox will be supported for spares and internal upgrades. Just because a gun doesn't look like an M4 doesn't mean it won't have the same internals. Granted some guns have specific parts (AUGs for example have a different cylinder head) but don't let that put you off. Buy something YOU like.