Well go to my site for the cheapest and simplest kind of build. Next to friction drives and the rag joint sprocket types, both are f&%cking worthless, the FWD I created is the cheapest to build though you have to weld a couple of plates to the forks and use have a drill and a grinder. If money isn't a big deal go with the IPS crank set up from J&B importers, that way you and your motor will utilize the gears on the rear wheel of your bike if you have them.
http://www.konasports.com/ips-road-r...crank-set.aspx