It depends on your budget.
If a CM16 is what you can afford, then you should definitely get one of those because it is the best gun available for that price range. As for it being too big, there is a shorter version of it. It all ends up being personal preference really.
If you have 4-500 to spend on a gun, you should definitely be looking at VFC AEGs.
NightFall is a harder event to get tickets to, you have to sign up and pay as a squad and it sells out in minutes. Don't worry, there are other games held at Picton throughout the year if you can't make that one.
Do not buy a full metal kit. It always ends up costing more, and you end up with a Frankengun, hours of frustration or a lot of tech fees. If you want a metal body down the road, I recommend you sell your G&G and get a VFC.
Prepping for a milsim is a lot more than just how many mags you have, its more about how to remain active for the full 18 hours. Think more about food, water, shelter, navigation, communication etc.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN