Hi
IMO I think the price of £5820.00 + £200.00 RFL is rather high for an extra row of seats and a padded bulkhead with 121k on the clock. It does appear to have cleaned up well though.
The other factor to consider that as rear cargo area is smaller and directly over the back wheels. This will affect the weight that you can put in the back. So with a standard van you would easily put a 650l tank in, because you would fit it closer to the front and hence spread the load between front and rear axles. The further back your tank is the less you can do this. so you would need to reduce the size, maybe to 500l or a bit less.
On some of these factory conversions, you could fold up the rear seat, like an estate car and move the bulkhead forward to give you more room in the cargo area. But the facility wouldn't help you with having a fixed tank in the back.
If you were seriously considering this, then I would phone Vauxhall Technical department and ask them what load you could put directly above the rear axle without exceeding its weight and work accordingly.
Spruce.