Diy all day long, why pay huge sums of money to a company to make up a system which, in essence, is about as complicated as a knife and fork. Tank, pump, r/o system, hose reels.
As for crash tested systems, i am not really convinced that there is any point. After all, a van is designed to carry pretty much anything that will fit in, be that a pallet of bricks or 6 motorbikes, so why the need to crash test a box full of water?.
Payload is a thorny issue at the moment, a couple of others have suggested a payload of 1500kg for a 1000l tank, however, based on my own weighbridge experience, and reading posts related to the subject on here, i would suggest you need a bit more than that!. Just recently there was a post from a guy with a 650l tank in a vauxhall vivaro or reanault trafic 2700. The weighbridge reading was 2960kg, more than quarter of a ton overweight!. Now as i said, that was the 2700 version, but he would still have been overweight in the 2900 version, which has a payload of around 1250kg. 1000l of water weighs 350kg more than 650l, so with that in mind, i would say you need a payload of 1700kg+.