Noob taking his 1st steps into this crazy world!!

[Infestormort]

Hey everyone,
So I've just got my first gun a G&G SIG 553, wanna start to pimp this bad boy up, not too worried about internal upgrades yet, just want to start putting sights, rails etc.
She's already got the top rail fitted, and it would appear that the drill holes are set for a rail set for the front hand grip, but I can't seem to find the right 20mm rail set on the zero one website?
So any advise on what to get would be appreciated so I can start slapping crazy crap all over this gun!
Also only one kind of magazine? A clear one? I've got the 30 round that comes with the gun, That the only one that I can get? Or can I go elsewhere? I got the 370 round extra mag, but wouldn't mind different types, what can I go for?
Thx for your time to read yet another noob thread, appreciated!