That sounds like a job for the Honda PCX 125 (our review of that is here: kit.honestjohn.co.uk/reviews/honda-pcx-125/). The maxi-scooter style of the PCX means a low seat of 764mm while the 125cc engine should give superb reliability and enough power for going up hills even with a passenger onboard. Also, being a Honda means it's reliable and there are dealers all over. The only other option I can think of with a lower seat is the Suzuki Burgman 400 with a 755mm seat height, but this is a physically bigger machine, so it depends on how happy you are with that and also if you have the relevant licence.

I think the seat height is a slight blind alley as it's more important to be able to get one foot solidly on the ground for stability at red lights. We recommend trying a few scooters to see what you like and going from there. If the bike is being left for a long period, fuel injection will be ideal as it won't gum up the same as a carburettor, but it's still worth putting in a fuel conditioner when it's laid up for a long period to avoid problems caused by ethanol in modern fuels. You should also look at getting a battery trickle charger to keep it in good nick and ensure easy starting when you return to the bike.