Originally Posted by BigBlue
Here's a couple possibilities. Staton is in the process of designing a shift kit for the Atlas cargo bike that he sells on his website. Another possibility is to use a Staton gearbox and a chain drive with a pusher type bike. Those training bikes that attach to the back of the parents bike.
You might want to consider going electric with a hub motor if you want quietness. Your costs are going to be higher, especially if you go with Li-Ion.
Here's a battery powered pusher trailer that is electric that can carry cargo. The guy sells plans. You could have a local welder build it for you. If you search the site, he built the trailer for the same reason that you had originally posted : http://electricbicycletrailer.com/
A 48 volt hub motor and some Li-Ion batteries and you could have some range with you pedaling.
That Sun Atlas Cargo is niiice! Looking forward to seeing Dave Statons 2 kit options!