Need advise on getting electric bike that goes at least 20 mph, 40 mile range?

What electric bike should I get?
I need it to go at least 20 mph and at least 40 mile range, I don't mind doing some pedaling to get long range. Also, I want it to be super silent.
Is it better to convert a standard bike or to get an already assembled electric bike?
What should I expect to pay for this?