I have been through this thought process - albeit in our last house. There is no immediate need but wanted a documented costed solution before it becomes forced through circumstance.
stairlift - simple, quick to fit, relatively cheap, but a constant visible reminder of infirmity
lift - difficult for many to find ~1 sq m vertically without seriously compromising house layout - eg: lose a bedroom. A smaller issue in a larger house. More costly than stairlift.
convert attached double garage. Currently mostly serves as workshop but if I get to the stairlift stage the workshop may be decoration rather than useful.
On balance I went with bullet 3 - power and plumbing are easily accessible, there would be no need to go upstairs. Existing bedrooms (4) would be unused bar visitors. No changes to kitchen, dining, conservatory living room. I hope it never happens but best to be prepared!