Do you want to power a front wheel or a back one? If front, the Wilderness energy motors are a good choice. I would recomend the brushless one though. I found the brushed one makes heat, and can cause the thing to go up in smoke if run too hard in hot weather. I live near Juarez, in new mexico and I melted one last summer. The brushless motors run much cooler, and go farther per charge. A very good deal on this type of motor, made by aotema, can be had at hightekbikes.com, and more info on it in the review and testing section. The aotema motors have the advantage of not having hall sensors. One less thing to break. They do make some noise starting up, but they do start themselves, even uphill.
Other very good choices would be ebikes-ca, where you can get chrystalite brushless motors. a 408 or 4011 motor would be a very good choice, and you can get those ones in rear wheel if that is what you need. The higher the number the more efficient they are at lower speeds. So a 4011 might be real good for you.
Geared hubmotors are the best choice for high power at lower speeds, hill climbing, pulling lots of weight. But some are very expensive, all make more noise than direct drive hubmotors, and some will strip the gears if oveloaded with current, above 48v. Ebikes ca's ezee is supposed to be very good, but above the price you wish. One gearmotor, the bafang, may be in your price range. Knuckles on the endless sphere sells them I think.
The 5305 would without a doubt be a very very good choice for maximum power, and speed too with that one. But again, more money than you are wanting to spend. You won't be sorry if you buy a 5305, but it will cost more.
Cheapest good motor kit may be the Aotema. It's what I run, so obviously I like em.
Best technical support is from ebikes-ca
If you go with a brushed motor, and a big switch, use 24 volts. The motor won't fry itself on that voltage, it might at 36v if used without a controller. Also the switch won't weld itself shut as easy. If that happens, you will need to be able to reach a plug connection to shut it off, or you will have to jump for it. Brushed controllers are the cheapest, so using a controller would be a better idea, and much safer.