Consider having a layout that does not cover spring hanger areas in case these ever need welding up if you plan on keeping it for ages.
That way you won't need to rip it all down, another reason why the bench seat/across van bed works well as you can hinge the tops for storage and also create open space over spring hangers for easier future welding. You can do the same for front rear hangers in yours which will probably be about where i would put a fridge above small pan cupboard on one side and cooker/sink above cupboards on the other running gas pipes underneath van floor on outside.
If you are just using a two burner and grill this could go on a ledge above a fridge on nearside, i lined my cooker/oven locations with ally lithoplate small stainless screws and bonded to ply for fireproofing, the pictured ally trim covers the edges of the litho and matt white surface laminate and is fixed last, and after any painting, with small round-headed stainless screws and top quality white silicone.
Use 12mm 5 ply formers for structural uprights fixing with 25mm sq batons, pva glue and self tappers and 6mm 5 or 7 ply birch for cupboards/doors/overhead lockers etc.
Travis Perkins do a good deal on 3m x 1.5m x 6-7mm birch ply sheet, i used two of these in the amby and scored the 12mm and boxes of screws from them also, their trade pack screws are still completely rust free in mine.
Best to use stainless screws/bolts for floor stuff from a good and friendly local architectural/building supplier or on-line.
Specialist fixings could be used to get a few lengthways structural fixing batons down/up the sides/roof. Or strategically welded fixings?
On-board water is on floor beneath oven/sink on mine but it's a big tank and you may as well keep it simple with a drop in 12v electric pump into an ordinary 25l water carrier which can be sited both in or out of van for use with a waterproof (boat chandlers) fag lighter type 12v socket outside under van combined with simple (cheap) grey coloured cover flapped water inlet from caravan supplier.
I store a 25l water carrier and waste carrier under each of the bench seats, ie' four containers, the seats have three compartments with 12mm ply formers and because of the angled fronts give seated foot room and allow a black waste with a fresh water on top in the rear compartment of each, the seat facings are 6mm.
Fitting a basic 2 socket mains kit will allow use of a micro type electric heater blower (charity shop £3, caravan shop 20 quid) and a small catalytic is handy if no mains.
http://www.mmleisure.co.uk/cgi-bin/site ... =mmleisureone of these is handy too if you're at home or on a mates drive!
http://www.mmleisure.co.uk/cgi-bin/site ... =mmleisureA simple but decent quality lekky drop in water pump in the boat chandlers round here is @ 10 quid.
Don't get hooked into water heaters, cassette bogs etc. keep it simple.