You will struggle to cope with just 500 litres of water a day with 2 people, so you would need to top up, but it seems a very expensive way of having a place closer to where you work to top up your water from.
In the other post, with your van photo's and so on, you say that you will only have someone along with you 2 or 3 days a week, so a rented unit seems an even bigger overkill

You don't say how much the rent is though, so I suppose that if it was little more than a peppercorn rent then of course it would be a bargain.
But if it is several thousand a year you'd be better off lease purchasing a second van and equipping it.
would be just as tax efficient and if you have the right system at home your water will be free.
Hell, you could always simply upgrade to a van that will take a 1000 litre tank, 2 people should cope perfectly well on that much water.
You say that you will be having someone work for you, that alone is expensive!
Employers liability and so on....so why add more to your overheads than you need to?
Ian