If all you need is 350ltr, then I'd highly recommend a VW Caddy.
I have a 400 litre pure2o system in my caddy, and imo it is the PERFECT size and setup for a one-man operator.
Caddy's are more expensive than other vans, but it's true what they say, extremely reliable, solid vans.
Mine's 2009 plate and still drives like new. When it finally gets too old I'll be replacing it with...another Caddy, probably brand new, as I'll be keeping it for as long as possible!
Superb vans, that is all.