Here are some parts you will need.
If you upgrade the spring, you will need to reinforce parts such as: spring guide, piston, piston head, reinforced trigger sear, and set pin.
For accuracy, I'd go for a tightbore, a new hop up unit (I hear the l96 hop ups are junk), some barrel spacers, a nice bucking, and all the little homebrew mods you can think of to get the air seal perfect and keep it as consistent as possible.
Tell him mGunner if I missed anything.