Re: diverter conversion?
Do you have an access panel cut out on the backside of the shower wall to get to the shower valve itself. How are you planning on cutting out the old and put in the new? I under you said fomica wall. In a tub/shower enclosure - intereasting. You might want to consider udating the entire tub/shower enlosure. The last I understood, formica is for counter tops.
Kohler is a good brand. You might want to try buying from a Plumbing Supply Store. The quality is better, but the price might be $10 more per fixture.
Good Luck. Process of Elimanation.
Process of elimination. Good luck.