Welcome to our community. My advice would be to use the search feature and look for answers to any questions you might have. All of your concerns have been covered numerous times but if you fail to find an answer feel free to ask.
35 to 40 is actually on the high end of the performance scale for a simple, unmodified motorized bicycle. It is not unachievable but 25 to 30 is a more reasonable goal for a new builder and a stock engine kit. As was said, weight does play into the performance but there are many builders/members here in your weight catagory who have little trouble attaining the speeds in the mid to high 30s.
Good luck and let us know what you decide to build and if we can help.
Oh, and if helicopters were meant to fly, they'd have wings