You're in good hands with Nougat's advice. Heed it and you'll be golden. What he told you about cheap e-bay sellers is all important. Don't let a few dollars dictate your choice of a kit supplier. Check into the reputation and warranties of the varoius sellers. After sale support is all important.
Also don't jump into this with visions of high performance right off the bat. Keep in mind that they're bicycles, not motorcycles and with your past experience with bikes you might feel that the performance is not what you expected. 25 mph on a bicycle is pretty darn fast for most folks, 30 can be scary. You're not going to pass traffic and in most cases you won't even keep up with it. Keep telling yourself...it's a bicycle, with brakes, bearings, wheels and tires of a bicycle designed and meant for speeds less than traffic.
Just keep listening to Nougat's advice and keep us informed of your progress.
As for gears...most of us never use them unless you have a shift kit. You pedal to start then forget about the pedals except for somewhere to put your feet.
Oh, and welcome to the forum. If you have questions we suggest you use the search feature at the top of the page. I guarantee you'll find enough reading material to keep you up nights while you wait for your kit to be delivered. have fun, ride safe.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"