I think it would work out fine, especially when sharing vessels within family. Your plan of changin out lower units while keeping the motor to fit the need of current and future boats is a good idea. I am planning on doing that with my 16' Gregor now; Changing out the LU to a Jet so I can use it in the rivers, and purchasing a new salt boat altogether.

In My family we have several boats, each individually owned, and we shared maintenance costs,help each other out on big jobs, and share when one wants to use the boat. We have a 58' Sea Ranger on Lake Union that I do a ton of maintenance on like changing oil, varnishing teak, general engine/ generator maintenance, washdowns, etc. It can be a hell of alot of work!