I'm in seattle. I live in Georgetown (a south seattle neighborhood surrounded by industrial areas, boeing field, the duwamish river, and I-5), so many of my destinations are close by and don't have much if any hills on the route.
I have 126V, 200AH unheated flooded lead in my 1985 MR2. That battery pack will allow about 50 miles at freeway speeds under the best conditions (temperature, elevation, wind, etc) but for running around town in the rain and winter with all the stop and go, 30-35 miles is max. That is using about 8KwH.
A 120V, 100AH LiFePo4 pack would have at least the same 8kwH usable as what I can get under less-than-nominal conditions for lead acid pack, and would also weigh about a third as much (my FLA's weigh over 1200lbs) which would increase overall efficiency especially in city driving with more stop and go.
Strongly recommend 120V, 100AH. Your mom won't have to go fast, but the car will be able to if she wants. If you must stay with 72V, use 160AH cells and design the car with enough room to upsize the battery pack to at least 96V or preferably 120V, for future upgrade potential.
Been to any SEVA meetings yet? (next one: march 8th)