I'd personally avoid the shift kit. I'm confdent that a good quality happy time engine could make the ride but 99.9% of my reliablity issues are with some component associated with a shift kit, e.g. chain breaking, repeated chain stretch and adjustments on drive chain to flywheel, OEM flywheel fails easily...get heavy duty one, torque affects on shift kit geometry causing issues with chain alignment, etc...etc... I love my shifter kit bikes but they are not as realiable as single speed bikes.
I think you best bet as a newbie is to consider a rack mounted system.