So you want to trick it out.
First, you need to know your budjet.
The reciever and stock that comes with it is fine.. if not you can change.
There are bunch of m4/m16/ar based sniper rifles.
Lot's of good examples in the Airsoft Media section, especially in the M4/M16 Thread.
You can get full metal front kits ranging around from 70$ to 270$.
Most front kits do not come with the inner barrel, so you'd have to put an inner barrel that would fit in the front kit of your choice.
Front kits are legal to import. A retailer like Huang sells front kits, like the SPR. He also as a variety of inner barrels.
After you are done with your pick, you would need to find a decent scope. Even though airsoft BBs cannot hit as far as the scope, a scope can be used as a monoscope to look for anything.
But since you are a noob, I'd suggest you to get out there and play a few games and get used to your weapon before upgrading. There are many upgrades out there, so choose wisely, and get ready to empty your wallet.