Used to have a 5 hp 4 stroke 22" cut and now have a 8 hp 4 stroke 26" cut (all two stage ie selfpropel) my father just trade in his 5 hp 4S 22" two stage to a Arien 7 hp 2 S 22" single stage because he is not crazy about the selfpropel units and my mother can't operate that in case my father was not home.
My suggestion is take your father in-law to the store and show him how's different models / features work and if he is mechanical handy and can operate two stage machine of course that's the way to go because all you have to do is switch it in gear and follow behind the machine but if someone feel like the machine is dragging themself then a single stage might suits better.
My 5 will do the job in Toronto but if you need to do it in a hurry (try to get to work and not being late) then 8 hp & up is better plus this is going to be a long term investment 5,7 or 10 years of use get the biggest motor you can afford and even few years down the road the motor is a little tired it still have enought juice to do the work.