I am just a lowely noob who plinks around with his gun, so take this with a grain of salt
I have a Jing Gong M4 S-System
and I think that it is an awesome first gun.
The thing is 80% metal, and it has nice heavy and solid feel to it.
It shoots hard and straight and there are tons of accesories that you can add on to it, if thats your thing.
I believe that it is a TM clone, so upgrading it should be possible.
I have never gamed this gun so I can say what the long term reliability is like, but if it is a decent quality, inexpensive, mostly metal gun that you are after, you can't go wrong with Jing Gong (JG) IMHO.
As far as a GBB pistol goes, I started with a KSC - G18C which is all polymer on the outside and I think that it is a pretty fair representation of the real thing.
The thing is cool because it shoots in either semi or full automatic modes. It is however a little finicky, so I don't know if you would want it as your primary pistol that you use in a game.
If I knew then, what I know now, I would have started with a TM - 5.1 Hi-Capa.