Just a couple more points:
We had out vehicle type changed on the V5, I sent a document to them detailing pics of the conversion and the spec, they sent the v5 back as motorcaravan, easy! I know a few people have had problems with this though. I can send you my doc if you need a guide, but theres not much to it, mainly pretty pictures!
400l + i'd say, and i'd also strongly recommend having it in the van. Underslung black and grey tanks are fine, but the last thing you want is a 400l water tank freezing so you cant do anything in the van, not to mention the fact it may burst and leave you with soggy feet. Same with water lines, try and keep them all inside the living space.
Not sure how big your seat boxes are, but maybe split the tank, have 200l under each or something, or build a 'water box' under the bed. Also, depending on the caravan, the standard water pumps can be ****e, is it a pressurised system of mirco switch? If its the latter, i'd recommend getting a shurflow (sp) pressure switch pump. See mine or ians thread for details on those.
Using the existing tank and pump is just added complexity for the sake of using the kit i think, keep it simples!