"Also, if you want the best sauna experience the ceiling height should be 7' or less to optimize the heat in the room and on the benches so you would want to drop the ceiling otherwise the heat just stays up high."

That's a recipe for cold feet. You don't want that.

What happens if you raise the benches up with the 8' ceiling?

Come to think of it— with the above plan, you could likely get by with having just a glass panel separating the (walk-in) shower area from the rest of the bathroom, meaning you’d only have to purchase/install one glass door to access to the sauna from the shower area.

How long do you estimate staying in the house? If not for all that long, then don't bother with a bathroom re-do -- you won't get recoup the investment. I've always heard the general rule of thumb is if you're not staying at least 5 years, then don't sink a lot of money into updates, and even then it depends.

Do you have another tub somewhere in the house? If not, don't ditch the tub unless you know you're staying put for a while. Lots of people like tubs, and young families with kids may have a tub as a requirement. I personally would find a sauna in a bathroom a negative, and I imagine plenty of other buyers would, too, so if you don't envision staying there for long = no. Sauna elsewhere, maybe (the basement?), but not within the bathroom.

Plus, it kinda doesn't make sense to me -- it gets plenty hot and steamy in the bathroom, especially if you don't turn the fan on, why bother with a dedicated unit?