If the gun currently has a plastic gear box, a stronger spring will definately increase the chances of its failure. A brand new version 2 shell can run you anywhere from 30 all the way up to 80ish dollars, and that would only help if all the parts in that plastic mechbox fit correctly into that new shell. Also Im not even certain that the dimensions of the gearbox from the stunt studios m16 are the same as those as a regular v2 mechbox, so there may in fact be some fitment issues.
It seems as if your funds are very tight at the moment, I would put the idea of upgrades or a new airsoft gun on the back burner for a while until things free up a bit. Enjoy what you have now, take good care of it and hopefully it will last until you can afford to purchase something better.
"Its only a little bit on fire"