Belt sander definitly but I would consider getting a Sharpmaker or Lansky first.
If all you want is to put a really good sharp edge on a knife all you really need is a sharpmaker. A belt sander is good for big reprofiling jobs like putting a fine edge on a YKLE. Even though I have a belt grinder I usually use my lansky because it won't let you screw up while you can make really bad mistakes really fast with a belt grinder. I even used my Lansky with a coarse diamond stone to do the reprofiling on my YKLE although it took about 16 hours of grinding by hand.

That is some serious elbow grease and definitely paying some sharpening dues. There is no fast and easy way to sharpen. It is an art and must be practiced.