i would say a 400 would be ok, i have a 400 in a doblo with similar payload, just whack some barrels in if you need.
what i will say though, is i had a caddy hire van when mine was off the road getting fixed and i put a tank in the back , it was great the caddy handels the weight better than any other small van i have driven and drives like a car, if you board out the back an upright fits between the bulkhead and the wheel arches , leaving the space infront of it for your real etc, i fit poles in across ways, but if it were my van i would cut a hole in the top left hand corner of the bulkhead just about big enough for a couple of slx's