i've got to disagree with Gearnut on a few points.
Coaster brakes (back pedal brakes) will work fine and last a long time, if you have a good hub and maintain it. my first bike i rode for months with nothing but a rear coaster brake and had no problems stopping. i think it's the cheap dept. store bikes that are giving coaster brakes a bum rap. a shimano or bendix (or a velosteel, if you can find one) are great hubs, and have been around forever, and have lasted the test of time.
that being said, couple that with a front drum (or disc) brake, and you have more than adequate stopping power.
i think the key thing to realize, is a rider moving at 35-40mph needs to rely less on his brakes, and more on his ability to avoid obstacles. Newton's Laws of Motion apply. mainly the first law. "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." basically, if you're moving along at 35+ and a car cuts right in front of you, the brakes may stop your bike, but not you.
obviously, you need good brakes, but you can get the best brakes in the world, and skid right into an accident. that's what i mean by "avoidance." you need to think that every car on the road is going to cut you off, every pedestrian will step out in front of you, and every dog wants a piece of your leg.
also, 2 stroke kits can be very reliable, no matter where you get it from, as long as it's maintained and set up properly. you'll have to pre-mix your fuel, which as you stated, you want to fill up at the pump, so a four stroke may be the thing for you.
i have no experience at all with four strokes, so i can't comment on them, except to say i think they're ugly. same goes with friction drives.
as a general rule, the better the bike, the better the ride. the difference between a $100 bike and a $500 dollar bike is night and day. and a $500 bike found at a yardsale for $50 is even better.
but maintenance is still key. if you ignore your bike, it'll let you know what it needs. usually in a disastrous way. the term "a squeaky wheel gets the grease" is meant literally in this case.
one of the joys of this "hobby" is how inexpensive it can
be. but it still has to be done right and taken care of.
and everything you need to know is available right here:Bicycle Motor Forum - Motorized Bicycle, Engine Kits, Manuals, And Help