[naminator]

1) What kind of build are you going for? There are various flavour's of rifle out there.
2) How long is your inner and outer barrel?
3) How long will you RIS/Silencer be?

The G&G is a full length M16. What I would do in your position is:

Upgrade the inner barrel.
Upgrade the hop-up rubber
Put some sort of optic on it if you want.

I had/have a bipod on my RPK and HK416. I never use either. They seem like a good idea until they get caught on stuff, make it uncomfortable to aim down the sights of your gun, get in the way ect. If you plan to be on you belly a lot a bipod doesn't hurt.

Honestly upgrading the accuracy of your otherwise stock gun and getting a good optic should be top priority. Once you have those you can start picking out rail systems, SPR kits and the like. Spending 150-200 dollars on cosmetics, then another 100-200 on internals to find out you hate that M16 would suck.

I got rid of my M16 after my first season when I realized how large and unwieldy it is. If this is your first gun I recommend trying it out before pimping it out.

Just my 2 cents.