How quick an acceleration are you looking for? Zippy? Or just pokey? Better than the quad gets?
If you're planning on a hubmotor, you can play with the http://ebikes.ca/simulator
and different motors and power levels (batteries and controllers), and see what kind of torque levels you'll get out of them at different speeds, which can give you an idea of how quick acceleration would be, and give you some ideas of combinations you could use to get that.
If it's only 180lbs total rider&bike, then you could get fairly zippy acceleration with 2kW+.
As an example, I have a bike that weighs (with me on it) about twice what yours will, and has about 4kw, 2WD, and reaches 20MPH from a stop in around 4 seconds. That's a bit less than "zippy", but it's still a little fun.
I'd make a guess that it'll be more like 8-10 seconds to 20MPh, depending on torque level, at only 800w. If torque is low, or there's hills, it'll be even pokier. :/
I'm guessing the bike you want to convert has a wide rear axle, probably wider than the typical ebike motors will fit. There are motors (like the QSmotors in the for sale section here) that would probably fit, but you'll need to measure the distance between the dropouts to make sure you get one that fits. You'll also want to get one with wide enough spoke flanges to be able to lace it into the existing rim.
You could go with a motor that drives the chain to the rear wheel instead, mounted in the frame, but then there's less (or no) space for the battery in the frame, which is probably the best place for it (keeps it safer when it gets crashed now and then).
Sprocket size on the motor will depend on the max speed you want out of it (or max torque), and the speed the motor is wound for at the voltage you want to run it at, and the sprocket size on the rear, and the wheel size. Smaller sprockets get you more torque, larger ones get you more speed.