Cool, I wasn't planning on taking a cart but it looks like we are anyway. Since Magen already has a donkey, I'll buy one, which gives us 3200lbs of space.
Magen and retainers with 1 months food each, two tents and 10 days feed require 385lbs in total. If Greenkarl is happy with the donkeys being ok with 1/3 feed and 2/3 grazing, otherwise I'll buy more feed. Either way that leaves plenty of space in the cart.
EDIT: I've also added a block and tackle, 2 barrels and some more rope.