I recently posted a thread about the SOCOM Gear R700 (which I am probly going to get) but just in case it doesn't turn out good, my second choice is the UTG MK96. On the site I found it on (Airsoft GI) there was only one upgrade: a 6.01 tightbore. Where could I get more upgrades (if there are more)? Is an L96 replica expensive to upgrade (I don't have alot of money and won't get much)?
Any upgrade from Maruzen will work in the UTG. If you just plan on trying out sniping, and don't want to upgrade immediately, than the L96 is for you. Pretty much any sniper rifle is going to be expensive to upgrade.
Maruzen rifles ---> Maruzen hop up and disassembly guide. Should have 99% of what you want to know.
On the other hand, with any stock rifle you're going to have to get down and dirty for good performance. At the very least you'll need to degrease everything, clean the barrel, and shim or tweak the hop up in some other way.
The nubbing (nub) is the buffer between the hop up arm and the rubber. SCS (shredder's concave spacer) is a good one. You can also mod your own concave nub.
Generally in the L96 a concave nub is the way to go as opposed to a cylindrical or H-nub. The PDI chamber's design isn't perfect but doesn't have as much room for modification as a vsr-10's (although you can do a decent amount) so a flat top-down adjuster isn't a great idea.