If you have the $$$ to spend, go with a Schlumpf drive up front and an Nexus 8spd red label internal hub and you'd have an ultra-wide gear range bike that requires minimal upkeep. I suggest holding out for Peter based on what you want and skip the steel one. Save the almost 2lbs of weght by going with aluminum. That will help offset the weight of the internal hub. Get a BOB trailer and the appropriate BOB NUTZ (fasteners for the trailer to attach on the internal hub) and you'll have a nice ride. I would watch going much lower than about 25 gear inches or you will be ahead to walk up the hill. I think you can get to where you want to be with Peter pretty quickly.
I like the dual drive and dislike it simultaneously. It has a wide gear range but I hate the little click-box that hangs off the side that seems pretty fragile. Another issue will be trailer fitment as well. You will have to go to the type of trailer that fits onto the rear frame area. The BOB trailer works very well on any of my bikes and you can get the attachments for MOST derailleurs and the Shimano Nexus and Sturmey Archer internal hubs too.
As for the knee have you tried any of the glucosamine complex type formulas to help with it?