I'm a freedom trolley fan. I have a van mount as well. If you get a freedom trolley it won't be wasted no matter what you read.
Get a water tank, submersible pump and you have a van mount.
You don't need loads of 25l containers if you have a water tank, just 1.
However, nwh and matt have given you cheaper alternatives which are just as effective.
It depends on what you want and how much you want to spend.