You really shouldn't have to buy a brahmin caravan, you should be able to build it, along with attaching a tamed brahmin or 2.
Anyone ever play that game freelancer? You could purchase quantities of certain cargo in one place, then transport them to another and sell them for a profit.
If we had something along these lines, the caravan carts could function like freighters, being very slow but having crazy carry capacity, whereas lighter vehicles, like the scout, cockroach and buggy could have reduced inventories, being used solely for player transport (with some gear).