I would suggest that regardless what brand engine you buy,do some pre-emptive work on it before taking it to her. Flooding crankcase with fuel cause's more engines to fail than wear and tear. If fuel is gravity flow,install 12 volt shut off that open's while key is on and close's when key is off. Safty switches cause 90% of no start situations so deal with them up front. If you believe the mower would be unsafe without one or more,replace them with more robust and reliable versions.
On a different but related subject. Be sensitive to her needs,respect and work with them. Older people want to feel self reliant but that doesn't mean they have to do every thing for themselves if you help in ways that doesn't make them feel inadiquate. For instance yardwork. First understand what they get injoyment from doing(flowers,fruit) and notice areas where they aren't as vigilant (grass in fence,weeds in alley,holes,mounds and rough spots,insect & pest invasion). Make it a visit while doing things for them. Based on my personal likes and dislikes,here's help that would upset more so than be appreciated. Two of you roll up unannounced and go at it with a commercial size Z-turn and string trimmer at a break neck pace. Mower set low scalping any place that isn't perfectly level.( I would interpet that as doing it so it will be longer until needed again) String trimmer with it's .130 line peeling bark from trees and scrubs. ( string trimmers do more damage than insects) Load up,rush in and say "we would like to hang around and visit but I need to go home and finish my own yard"and out the door and on the road you go. I would be so uptight I would be much better off with an overgrown yard. I don't have the gift of being able to explain it but all isn't as it appears with elderly people so try and put yourself in their shoes. I leave you with this to ponder. Your parents tolerated a bunch of crap from you as a child,try and be as understanding and loving of them.
