There are a large number of choices in the electric bicycle world, many "fly by night" companies. Some are truly excellent, some are awful.
I've ridden quite a few, as I had intended on building my own design (again, due to knee problems) . A 500 watt motor is a bare minimum IMHO, as that's enough to climb hills and accelerate with more than a trickle of increase. What I quickly discovered is that sitting on a bicycle, and operating it like a motorcycle is very uncomfortable. It's not like riding, where you are always active and pressing on the pedals, and constantly unweighting one's behind, allowing for good bloodflow.
I typically ride between 14 and 17MPH on my bicycle, but on a powered bike, that seems too slow. So I tend to go quite a bit faster when on an electric.
So, I tend to like Mountain Bike type electric bikes, with the feel of a dirt bike and some suspension to cushion the "faster" ride.
I would tend to like something like this: