First I'll say that the Sig 552 uses a V3 gearbox. By design of the shape it's slightly stronger than a V2 gearbox but that's not really saying much since part of the strength comes from design and some of it comes from the construction materials.
Now that that's cleared. I would start worrying about cracking gearboxes and stuff after you start going with a M120 and above springs. You can get reinforced V3 gearboxes if your stock one does crack so don't worry about that too much (should run between $40-45 + shipping if it's in country).
As for charger go with this combo (you might be able to use a laptop power supply to power it, but might as well buy the power supply at the same time while you're at it so you don't have to share one between your laptop and the charger):
As for the battery, I know the Sig has a pretty tight compartment, but maybe one of the Turnigy Nanotech "airsoft" ones from Hobbyking will do the trick. I don't want to recommend any specific one and have it not fit so take a look and see if you find one that has around the same dims that you're looking for.