A battle belt would work fine for what you want to do, you could buy the harness or just get some cheap suspenders and you would be set after buying a few pouches for it.
I would setup my rig like this-
Battle belt w/harness
M4 double mag pouch[or any type of doublemag pouch] On left side
Radio pouch on left side
Utility pouch[such as a medic pouch] on right side.
You can also throw on a Molle pistol holster or just a dropleg for your sidearm.
Then throw on a camelback and your set.
This would provide enough storage for you to carry mags, bbs, maps and your radio while still making it easy to lay prone
You would still have extra molle on the front so if you decide you dont want to be a sniper, you can add more pouches for an assault loadout. This is an overall pretty low price rig if you shop around right.