Hi Tom-- Before doing something like this, you want to check that entire floor, and make sure there are no cracks in either the tile OR the grout. You also want to take something like a butter knife, and tap that whole floor out to make sure there are no hollow spots. So long as everything comes back the way it should, then take a belt sander and rough up the face of the tile to give the thinset a little more to bite into. For thinset the ONLY thinsets I'd recommend are unmodified thinsets mixed full strength with a liquid latex additive to make sure you get a good strong bond. Something along the lines of Laticrete's 317 thinset mixed with their 333 additive, or Mapei's Kerabond thinset mixed with their Keralastic additive, or Hydroment's Tilemate thinset mixed with thier Flexalastic additive.