I never followed up on this thread...
We bought a lift on-line shortly after I posted this. Company provided a worksheet for stair length, tred, and landings. Filled that out to get the right length and mounting parts. It was a simple install; took a couple hours. The lift works well. Fortunately, the stairs are ~4' wide, because the lift rail and assembly to take up space where you would walk.
Unfortunately, my MIL had a debilitating stroke a year and a half after this thread started. She is in a care home now, requiring more care than we can provide at home. Due to a fall when she had the stroke, she had to go out on a backboard, with full c-spine precaution. We actually rested the backboard on the stair lift to get her down the stairs. There was no way to get the gurney up/down the stairs. I'm a local volunteer FF/EMT; one of our Engines and Medic units responded.
My MIL got 1.5 years of good use from the stair lift. We have not removed it yet; not sure what we want to do... We're renting the Granny Flat to a friend to help pay for the care home. The lift is out of the way, so we just have not done anything with it yet.
I would have to dig in to records to remember what brand/model it was. I think it was Ameriglide.