I am also 70 and over weight, my idea is a mountain bike has lower gear ratios and suspension, so easier for me to push and will take my weight, but also a little heavier.
I was waiting to go to a shop to try some, when I saw a local advert where some one had bought an electric bike and now could not use it due to MS, so I bought this second hand bike which nearly ticks all the boxes.
Now I have googled the bike seems not that good of a bargain, paid £750 for it, but had 1 km on the clock. But all bikes in some way are a compromise, so what I wanted was:-
Folding so would be secure in the car - large wheels so no balance problems - hydraulic disc brakes - at least front suspension - not too heavy - prefer centre motor - easy to use gears. Well did not get hydraulic brakes but are disc so need to watch adjustment, did not get centre motor but in rear wheel seems to work OK, did get large wheels 26", did get folding, suspension front and rear. Weight not too bad.
As to hill climbing I live in Mid Wales, so some steep hills, I would say 3 MPH is lower limit, any slower hard to balance, and without the motor I could not maintain that speed, with the motor I can with most roads, but some times no option one has to get off and push, being able to fold pedals is important, don't want them hitting your legs when walking, and also some motor assistance when pushing. The twist grip really is not ideal when walking alongside the bike, but neither is the assistance method with my wife's £1,750 bike, if you have to walk because too steep then likely only want 2 MPH maybe less, it is permitted to have up to 4 MPH, but that is too fast, the walk assist is also used to help restarting on a hill when not centre motor, centre motor as soon as you press on pedals you get assistance. Many cheap hub motors need to do 4 MPH for them to kick in, so need a method to get help.
It seems the controllers have options so you can adjust when the motor cuts in, and my bike has micro switches in brake levers which stop the motor, but wife's bike instructions says it has walk assist but never found how to switch it on.
The other thing is height of frame, when I was a lad I would stand on the pedal and cock my leg over the seat, today at 70 I have to lean the bike over and step over the frame, ladies bikes have a lower cross bar, so easier to get on and off, but many of the modern bikes have lower bars, which I completely mess up by putting my tin of tyre repair glue in the water bottle holder.
This is also a point, back rack can make it harder to get on/off the bike, and hard to fit on many mountain bikes, so I end up with a back pack, never considered where the battery was when we bought bikes, but mine on front handle bars and wife's in rear carrier, the latter seems to mean harder to carry the bike as too heavy at rear, not helped by saddle bags.