You can claim all the leasing charges back and you can also claim the vat back if you are vat registered, if you are not then you lose this.
The main problem with leasing is the balloon payment at the end of the contract, this can run into thousands so beware.
You could try instead Contract Hire, the deposit is normally cheaper and there is no balloon payment at the end of the contract, all you do is either hand the keys back or order another new van.
As for having the water tank bolted into the cab, this varies with each leasing firm, you will need to check, also make them put this in the contract in case of any disputes at the end of your contract.