thats suprising 650kg payload with a 350 tank would have thought he was under ?? .......... not saying ur wrong just always thought these were ideal tools for a 1 man operator.
1st, when they came out, they had a 610kg payload, i believe they are now 660kg.
2nd, the guy i am talking about had a proprietary system fitted, which was probably heavier than you could do a DIY setup.
3rd, the guy i am talking about had it set up for 2 men.
So the upshot is, you MAY be able to get away with a 350l tank in a bipper setup for 1 man, but you would have to be careful!. Ideally though, if you need a 350l system, it might be a better idea to look at a partner (850kg version), that way, not only would you be well within the van's limits, but you would have more space inside for your stuff. The difference in running costs between the 2 would be minimal, and you also have the option of 3 seats.