Friend of mine has a 400 liter upright tank in a Combo, seems to be fine. Not sure what the exact payload is but I think its 600-650kg. I'm not a big fan of the Combos, they seem to rust really easily. I have an S reg (1999) Peugeot Partner with a 400l flat tank in and its just right. Good van too, not a spot of rust on it and the engine is smooth as anything.
Why are payload weights so hard to find and so vague, when they're such an important figure?!