Thanks everyone for your advices it really helped me before getting my first rifle. I know a couple of people said that G&G is crap but I've also seen good things about this company.
So I just bought a used G&G TR16 TopTech R4 Commando for $200. It came with a 9,6V / 1600mah battery, a smart charger, a sling, and a front grip. Apparently the person bought it last summer and used it around 4 games. I have to say that the body is still in pristine condition and there is no scratch. I did a couple of indoor shooting and everything seems to work fine. Can't wait to test it in a game now!!
Also I have played airsoft for a full day last weekend with a rental AEG (G&G CM16 Raider-L) and i absolutely loved it!
Again, thanks to everyone that answered my questions and gave me some advices!