Nifskope or sceneviewer are what you would start with for messing with nifs. Most people use nifskope because it can do so much more than scene viewer. The only advantage of sceneviewer really would be its viewport. Which shows the unit exactly as it will appear in game (correct position, and any shader/gloss that it has). You only really need one of these for simple head or weapon swaps, and I would suggest Nifskope. Most of the time I use both, sometimes just one or the other, or simultaneaously, for various reasons. But for a beginner just using nifskope should be enough.
For painting its skin I would suggest GIMP. But any paint program that can handle DDS or TGA will suffice.