Once you get all the old stuff cleared away, the slab may have noticeable staining and/or a mottled, irregular appearance -- perhaps in a pattern -- due to bleeding from adhesives or even from mortar. I'm not saying you shouldn't do this. Just that you might have to plan on paying for the top surface of the slab to be ground down slightly to get rid of old staining before you can apply a new, decorative stain and have it look good.