I would recommend chair height at 18 inches. this is where we position ours unless there are some special needs. 18" is ADA compliant for toilets as well, so makes sense to install at 18". I would also consider a large seat/shelf. We install a lot of those as it keeps the floor area a bit bigger, keeps yo0u from stubbing your toes and makes the glass a little bit more affordable as the cut out tempered glass panels you see around the bench are more expensive then rectangular.

Some very helpful advise 'thank you'

No problem, also a a contractor i have replaced many of the old style shower pans that use the rubber liner and concrete shower pan. I have stopped using these and exclusively use the Schluter/Kerdi membrane system. It is a waterproof membrane shower system and is great! I actually wont install the concrete and liner system. You can watch a video their website on how it works and might want to ask your contractor if they have a tile installer that is familiar with it, thanks;.

Aw thank you very much for this information :)

18" is a good height for a bench seat in the shower.

Any way to affordable install a shower pan that is not ugly fiberglass or acrylic? We don't have the $$ to tile the floor, am hoping there is an alternative.

Unfortunately I have not found any more affordable then the fiberglass and the solid Corian style and the problem with those is they can yellow, peel, and degrade over short periods of time. I was actually with someone recently who had a fiberglass one installed in 2007 that has cracked and is now leaking. The most secure we have done is the tile ready Pre-fab solid pans or the Schluter/Kerdi system. My questions is, in saying you don't have the funds for the tile, is your shower leaking and in need of repair or is it a situation where you could continue with what you have and save the additional money to tile the pan during your remodel? If you can I have found doing it the more expensive way is better over the long run and a better investment of your money in your home and might be worth the wait to get started.