[Ibby]

You should get metal bushings. That will be pretty much all you need to replace internally along with the spring. A new gear set is a good idea but not necessary right away. I would order a spare set sometime, just to be on the safe side.
The half teeth/full teeth thing applies to the piston. Some torque up gear sets require some of the gears to have a different diameter to get the added torque. In some cases larger gears won't fit without making space somewhere, and one place is to shave a bit of material off the piston. Unless you're getting a bigger set of gears, stick with full teeth pistons.
I'm back in Kingston the first week of Jan. If you like I can help you out upgrading your gun then. I have a chrony so the gun can be tested as well.