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.