+1 on Nexus. Choose your front chainrings carefully. You probably wouldn't even need more than two. If you haven't checked out Sheldon Brown's website, he has some great info about the Nexus 7/8 that will help you decide how to set it up.
FWIW, my setup is a single chainring in front and Nexus 7 in back. I don't know the size of the chainring off the top of my head, but it is "medium sized." I ride it to Costco and back with a hill in between (uphill both ways ;D) and I don't have a problem with 50-70 lb loads.