The new shapes come in 850kg as well. I have a 400 litre tank in a 800kg Berlingo no problem at all.
That's right, you can get payloads up to 850KG with the new Berlingos. However, most second hand or new ones on "special offer" tend to be the 625 version! 
We are talking Citroen here.
Unfortunately dealers were required to commit to ordering stock from the factory up to 3 months in advance. Other dealers could then draw from your stock whilst they waited at depot to be called off by your dealership. If no other dealer requested your stock after a certain length of time, your orders were invoiced and shipped to you.
So dealers only ordered the basic most popular spec just incase they got landed with a whole lot of stock.
However, a few years back the special deals where identical on both the 600 & 800 models and lasted about 3 months. So find out what the deal is at the very beginning of the quarter and order a factory model to come within that quarter. Also dealers have a stock holding inventory of all the other dealers in the area using the same storage depot, so they will be able to source your requirements somewhere if they aren't lazy. The only time you may not get something is if you want a colour, as every dealer will order the most popular colour - white.
The other way of holding stock until the last minute at the depot for a potential order was to draw up fake orders against each of them. So a dealer may not be able to find a particular model and say they have checked other dealers stock. But don't be deterred - phone all the commercial dealers within a 50 mile radius and see what they can get - and don't forget to play one up against another price wise.
The only time the factory got involved with extra special pricing was a preregistered effort to gain market share at the end of a reporting year or number plate change.
Ford will usually come out with some good pre-registered delivery mileage only deals in October of each year. Citroen were more flexible but again usually at a plate change and would be what they were overstocked with ie 625kg models.
Spruce