I would just like to throw a few ideas out. I totally disagree with Cavi Mike's comment. Multi speed cruisers have a few perks over single speed
1. Multispeed come with front and rear brakes already
2. You can pedal past the motors top speed if speed interests you
3. No need to modify coaster brake
4. multispeed gives you the option of ghost pedaling with some resistance for exercise
5. Most important to me, you don't have to worry about pedal position when you come to a stop. with a multispeed you can spin the pedals backwards. With singlespeed, you have to roll forward to reset pedal position.
One particular bike I have personally motorized and really liked was the Fito Marina from bikebuyers. Even comes well under your $300 limit including an led light.
Just my .02, nothing wrong with seeing all options.