For a 1000l tank you want as big a payload as you can get, so i'm glad to hear you are looking at the crafter, and not the transporter!.
If you are thinking of new, then i would suggest looking on autotraders new van section at some of the 'less fashionable' vans, such as, citroen relay/fiat ducato/peugeot boxer, or the nissan nv400/renault master/vauxhall movano. One example which caught my eye was a mwb 3.5t citroen relay with the 130bhp engine for under £13k