Some Aygos did suffer from water leaks - mainly into the boot or front foot wells - so this is one thing to check when looking at cars. Most are fairly easily fixed.
One thing to bear in mind as regards a three door, whether Aygo or Yaris, is, as the doors are longer (and heavier), they can pose access issues in car parks, etc and if your mother garages the car, as in these circumstances, they won't open as wide. Could be one thing to consider as your mother is elderly.