The most important part of a Van is a good foundation on which to build your Van on.  If you have a good floor you can attach cupboards to it for extra strength.  So grab a tape measure and workout how many sheets of Marine Ply you will need. With this in mind I went for 12mm Marine Ply as it is waterproof and makes a good drill in structure (Thick enough to hold screws without touching metal floor). I was able to just use 3 large sheets and used the existing floor covering as a template for new floor.  I also made use of the existing anchor points to assist in locking the floor down to the metal van floor and used floor adhesive to give extra strength.  Once all Marine Ply sheets where traced I simply used a Jigsaw to cut-out my floor.  This was an easy process and I even removed the step trim in order to reapply it once the new floor went in.  It makes the job look a little more pro to use existing Van mouldings.  Finally I silicon the edges to ensure no water spills would seep under the flooring.  I also used a round door lock saw to cut out the existing circle anchor points on the wooden floor.  A nice coat of Floor Varnish and it was job done.  I bought the Marine Ply sheets from Gumtree.  The time to pickup was about 2 hours travel.  As Bunnings did not have large enough sheets and where over priced anyway.