Sounds like a fun project!
Other than the normal issues w/any sidecars (handling & maneuverability, width) the only issue I can think of is applied traction (minor), while there may be some "torque steer" (wrong word methinks, but I can't think of one better either lolz) from the offset drive wheel, I think it'd be minor & easily compensated for *shrug*
A couple few of our members have gone the sidecar route (tho none powered as far as I know)
http://motorbicycling.com/members/fasteddy/ &
http://motorbicycling.com/members/silverbear/ come to mind... Might wanna check this thread fer inspirations ifn you've not seen it already:
Sidecars?