In my view it depends on how you intend to ride it.... if ya racing the bugger be prepared to put a lot of doe and time into her! But if ya just standard riding around but want to give her a real squirt now and then then a Stage 2 kit should be more than enough to get you flying! Stage 3 is a big step up! And requires a lot of work to get it singing!
Go with a stage 2.... It will be the best! It should set ya back no more than $250.... The Stage 3 is a $100 more....
But remember the Gpmx3 is a pretty standard pocket bike.... I to have one.... I intend to 'practice' on it until I have the recourses to step up and race....
Good luck bro!