Perfect plumbing van will be:
One to carry lengths of 3m pipe inside the body (e.g. bundle of 15mm, bundle of 22mm and 3 or 4 other pieces).
One to carry 3-4 sheets of wood/plasterboard.
Easily access (near door) cubby holes for keeping small bits in - e.g. washers.
Flexible storage area - ie main frame with adjustable shelving and vertical sides and various straps to hold things.
Easy ladder storage in top of van
Space to "walk" in van for when it's raining
Possibly a small cupboard so you can keep home purchases clean for the "While you're out do you mind buying me a ..."
In cabin, decent sized compartment for folders, paperwork, brochures, etc
Decent space for sandwich box/thermos/drinks/crisps, etc
Rubbish bin
Proper space for phone - e.g. angled shelf/holder so you don't have to touch it to see if it just rang
"Part time" 4 wheel drive
Automatic gearbox
Heated windscreen
Air con
Electric windows
Seat warmers
Proper heater
Comfortable seats