If at all possible use a bike with full suspension, front and rear.
If you want a chance at hitting 50mph, get the shifter kit and expansion chamber from:
Sick Bike Parts LLC - Performance parts for your motorized bicycle
Those motors are fun but they are frustrating and need constant maintenance. I had one like that and now have a Dax Titan 50cc four cycle on a cruiser bike. I would like to move my Titan to a stretch limo bike with some upgrades to the drivetrain and a full coroplast fairing.
I find 30mph to be somewhat frightening and usually keep it down to about 20mph. However I am driving in town, not on open roads. Be aware that bicycles tend to shed nuts bolts and parts quite readily when ridden at high speeds with a vibrating motor.
You should be able to do much better than 100mpg. I think the time I measured a tank I got 120mpg (with the Titan four stroke).
Using the Sick shifter kit is important because then your bike can freewheel coast. I don't have that kit and my drivetrain causes a lot of drag, its like having the brakes on if the throttle is not on. With a good freewheel, some pedal assist and EOC I bet I could get 200mpg.