I'm 5'7" and I just measured my lower leg to see if it was 18". It is just under 18". If I were to step over the 18" tub I'd have to get on my tippy toes. If you never take, but the occasional bath, I'd go with the Villager as Writersblock suggests. If you don't like the Villager style, the standard alcove replacement Bellwether has a nice modern look yet is only 14" high like the Villager. Easy to step over, and it can still provide a bath:)

If you go for a brand like Grohe, you would order each piece of your setup individually. The handheld showerhead, the metal hose, the wall union, and the and the showerhead holder that attaches to the wall. The plumbing would be the same as for any shower over tub-filler with diverter, except that you mount the wall union and showerhead holder down within reach from the tub. That being said, Grohe makes a coiled metal handheld hose that's only 59". I don't think you'd want one much shorter, or you might have trouble maneuvering it to where you wanted. When the handheld wasn't in use, it would be a simple matter to simply loop the hose over the handheld resting in its wall holder. Here is a link that might be useful: Grohe 59

Do you have a specific tub in mind? We have a Bain ultra air tub and I couldn't imagine showering in it every day. The acrylic bottom along with the added height of the side.... to me it seems like an accident waiting to happen. I guess it would depend on the height of the tub you had in mind. In regards to the automatic air purge, it only happens after the air cycles off. You wouldn't be turning the motor on when you shower so the automatic thing wouldn't happen.

I have a bathroom with a very low ceiling of 81". The height of my tub is 13" and from the top of the tub to the ceiling is 68". I'm looking to buy a semi-frameless glass bypass door that is tall enough that a 6' person won't have to play limbo to get into the shower, but also low enough to allow an adequate ventilation gap at the top. Seems like the Lowes/Home Depot standard is around 58", but that seems short. I've seen a 62" tall door, but would a 6" gap at the top be enough for ventilation???