Being an ebiker for the past 4 years I can tell you 40 mph can and is done. However, as so many have told you here already you have to be half (or more) to go that fast on a bicycle. I will go 30 on mine but have no interest in going any faster. As the saying goes "speed cost money".....how fast do you want to spend? If you really want to do 40 you best have some rreally great brakes and one hell of a bike. If you hit one little stone you'll be nothing more then a red splush spot on the pavement. trust me and keep your speed under 30mph.

As whitecat correctly told you the bettery is the most expensive part of the ebike setup. I carry twin 24v 20ah lifepo4 Thunder Sky packs. Total investment for the packs, LVC alarms and chargers is close to $1,000. You could also go with a lipo pack for less but it adds a little risk and is not for everyone especially a noob.

Winter is coming so take the time to do more reading and for sure check out http://endless-sphere.com/forums/ and even http://www.ebikeforum.com/ . You could start out low cost with an EZip bike under $400. It's a complete ebike not a kit. My wife and I each have 2 ebikes and we both have an EZip. It's a good starter ebike starter for low cost but also low speed as if comes from the factory (16mph). However, there are a lot of ways to increase the speed and distance of this bike. There is a lot of info on it at http://www.ebikeforum.com/ . But as I said speed cost money but at least this bike gets you into ebiking at a low cost and you can build on it as time and money allows. The one negitive is it's a little louder then most hub motor (geared on non=geared).