How to make an uneven, damaged 'rustic' tile floor even for tile & parquet laying
Unless you are concerned about an asbestos material under the tile (such as old linoleum squares), I would remove any old material and start clean.  If you keep building on top of the old materials, you might need to rip all your doors down to make them fit with the new floor.  Conversely, you need to consider how high to make the new floor in relationship to the old floor.  Last, while you may still need floor leveler, a better bet is to install a nice new clean level underlayment board such as concrete board, durarock, hardibacker, or other tile board for tile or a nice clean 1/4" sheet of plywood and paper for hardwood.