My Simplicity mower originally came with an 8hp engine. My electric setup is a Motenergy ME0708 motor running at 48V. It typically pulls 70A during normal mowing and 100A going through real thick/tall grass. At 70A, the motor is putting out roughly 4.5hp. So, during normal usage, the gas engine was only using roughly half the horsepower it has. At the peak ratings I've seen (100A draw), it is still only pulling 6.5hp. The ME0708 is rated for 6.4hp continuous and 20hp for 1 minute. So, I'm well within its rating and I'm sure it'll last forever.
Before I got my ME0708 I was using a series wound dc Baldor motor rated for 1.2hp. It lasted about a year and a half before I ended up overheating it. If I would have added a fan to that motor I would probably still be using it though. So, you can get away with a much smaller electric motor. Is it harder on the motor, yes. Does it work, yes. How long will it last? I don't know.
So, as a conservative guess, I'd say you're generally alright going with a motor rated around half the gas engine's power rating. For your purpose, an ME1004 (which is basically made to replace small gas engines) or ME1003 would probably work out great. You could get away with an ME0709 probably, but for the minimal price difference I don't think its worth it.
I like the PM motor for motors better for the mower. The rpms are much more constant when the motor is loaded vs not as loaded. With the series wound motor, it would speed up when I disengaged the blades noticably. It is hardly noticable with the PM motor.