Converting your regular bike into an electric vehicle with the use of a conversion kit may be the easiest and least expensive way to go, without having to buy another dedicated bike. There's no shortage of kits out there, and the db-RevO from Taiwanese bike company DK City allows riders to make the transformation by simply replacing the front wheel.
The db-RevO retails for about US $1,170, which is definitely on the more expensive end, but with it may be worth the ease of installation and the aesthetic plus of hiding all the parts in the wheel itself. Theft may be a problem though; you'd probably have to carry it with you or lock it safely somehow to prevent it from disappearing. Check out the website for more detailed specs and ordering.