From what I have heard you would have to get whole nee barrel set in order to install a rail system into it. I heard that from somebody whose been using his G&G M16 for over two years.
On the other hand, why dont you go to a game before jumping into buying an AEG. That way you can :
1. Get to hold lots guns, and see which style you like best. In my experience most if not all airsofters love to show off their babies, and tend to be very friendly and helpfull.
2. You will have a chance to ask around where to buy stuff from. Trust me, that will save you lots of money and will get you much better stuff. Specially when buying guns.
3. Most likely you will get the chance to get AVd.
I wish I had done that when I was a newbie. Instead of bought a cansoft G36 that I ended up hating