A plumber is ethically required to do this to code...a three-handle device is not to code. WIll you find someone that will do it, probably, there are a bunch of them whose ethics may not meet standards. You'll find lots more choices in code approved valves. With good access, it's probably not more than a couple hour job...depending on local rates, you should be able to make a guess yourself.
Kohler makes some nice stuff, but they are IMHO, saturated with NIH syndrom and I can do it better freaks. As a result, their parts book is HUGE, since many things don't stick around for long. This can make it difficult to find the proper part, and may be impossible if you didn't save the original instructions and diagrams with part numbers. This, as opposed to say Delta, that uses a few internal designs and parts are readily available both OEM and aftermarket (since there's such a large quantity out there it makes it reasonable for an aftermarket supplier to take notice and provide parts).
