Take the guesswork out of it - I always do it the other way round. Some where on the vehicle will be a plate giving the Gross Vehicle Weight. this is usually under the bonnet or inside the doorshut.
The GVW is the maximum the vehicle can legally weigh - obviously including all the equipment, fuel, driver etc etc as Alex said.
Fit your van out, put everything in or on it that you will need including a full tank of fuel, take it to the nearest public weighbridge (in the phone book) and weigh it with you sitting in it. The difference between the recorded weight and the GVW is how much water you can put in it.