Version 2 gears would work but for the most part a lot of A&k m249 users don't bother to upgrade since the stock gears are able to take an m130 spring with no problems. I say keep it stock until it breaks on you.
As for rate of fire, mg's are normally a slow firing gun. I personally don't want to increase my rate of fire even with an upgraded spring since it's already a bb eating monster when you are the fire support role for your team. But if bb's are not a problem I would suggest to start an upgrade with a 10.8 batt then a better motor if you want more action. Here is the other thread that we discussed about upgrades for this gun.
http://www.airsoftcanada.com/showthread.php?t=54355
Regarding the bamboo bucking, I you want to go this route, try to get a hard hop-up rubber. I softer ones might get damaged faster since the rubber will be pushing in to a much more solid object. That's the reason why buckings are made out of rubber, elasticity to avoid tearing of the hop up rubber.