Logistics of a Animal-Drawn Caravan

I've got a fantasy story in which long range travel over potentially dangerous terrain is a huge part of the setting. In particular, I've got salvage teams moving about trying to grab material and technology from the old world. I'm thinking that the main means of transportation would be carts drawn by a creature functionally similar to an oxen or similar draft animal.

The requirements for these caravans:

• They need to be able to provide for all their members for several weeks at minimum. The distances being covered are measured in hundreds of kilometers. The draft animals are capable of foraging, but the human members of the caravan will have difficulty procuring food and water out in the wild.
• Human members of the caravans are unable to venture very far from them without risking serious injury or death, since they hold devices critical to survival in the wastelands they're travelling through. The radius of protection is about 10-20 meters.
• They need to be able to protect themselves from attack. There are primitive nomad tribes who will occasionally attack passing caravans with the sole goal of exterminating everyone in them, as well as hostile wildlife. For weaponry, they have guns (with limited and precious ammunition) as well as lower tech weapons like crossbows, swords, spears, etc.
• They need environmental protection. Direct contact with freshly fallen rain is extremely harmful to humans in this wasteland (though capturing and purifying it can yield it as usable drinking water) and wind storms in the area are similarily dangerous. The people in the caravans need to be able to shelter themselves from the elements quite thoroughly. The draft animals are resilient enough to not be concerned.
• They're going to need spare cargo room to put hundreds of pounds of salvage which will include everything from old fuel to broken (but repairable) machines.

From that, what will these wagons look like? What kind of supplies will they need to bring along? How many draft animals and carts will be required? And how many people would you expect to come along?

I'm looking to send out a group of 4 (my protagonists) as a single salvage caravan. They're meant to be short staffed (even to the extent that their performance and capacity for self-defense suffers) yet still feasible.

I recognize that there's a lot of wiggle room here, but I'm interested in any information you can give me here to make this consistent and believable!

That's some really good information in the wikipedia article. I think I'll be going with 3 wagons for 4 people based off the prairie schooner, with about 6 animals per wagon. Assuming about 2500lbs of carrying capacity, that means there's plenty of room for provisions, equipment, and other supplies, but still tons of room for salvage or for losing one of the wagons.