Well, for $15-18k you'll definitely be looking at a 2000 Xterra, not a 2002 (esp. supercharged) if you want 4WD. Don't bother with the XE/SE, the only real difference is the sunroof and the stereo on the SE. Pretty sure the 01 added steering wheel audio controls to the SE, but otherwise there aren't any other exclusive differences other than minor appearance items (leather wheel/shifter, gauge color, etc.) that I wouldn't quibble over. Of course, the SE has all the goodies like cruise, limited-slip diff, power doors/windows/mirrors, extra power outlets, and an alarm, but those are all found on fully-equipped XE models too.
The Xterra will do everything you listed, and will do quite well in snow with a good set of tires. 31x10.50 BFG All-Terrains or Dunlop Rover RTs are my 2 recommendations for excellent all-around performance with very good winter performance too.
Oh, and it's not a car, it's a truck!
Brent