[Boy Blunder]

1 & 2) Popular character (possibly the most popular in the X series, depending on who you ask) in his most recognizable form (rebuilt version with Z-Saber). Vile has a cult following, but Zero is a definite Face character.
3 & 4) Keep if you like it, sell if you don't. But the whole value thing is goofballs. Short of a few items, collecting for the sake of investment is, at best, a crapshoot.