Hello, Mr. Love.
Congrats on the van purchase, buy please tell us which van you are acquiring.
To protect the load space, you might consider deodorised fish oil, available in bottles and spray cans (
www.bunnings.com.au/search/products?page=1&q=fish%20oil&sort=BoostOrder&pageSize=60). Use the bottle and a brush to apply the liquid to floor and about 50mm up the walls. Lay the load area mat that came with the van onto the dry fish oil. Use the spray can and long nozzle to squirt into areas in doors, tailgates etc, where you generally have to remove rubber grommets to get access.
If you are going to keep the van for a long time, suggest you ventilate the load area by using a ram air vent (
www.uesint.com/product/muller-vent/) at the front of the load area and a rotating vent at the rear (
www.uesint.com/product/rotary-vent/). Use only stainless steel screws and metal-compatible silicone sealants when affixing components. When the van is stationary, the vents allow air circulation and reduce salt-laden air from wet sails, board foot straps, wet suits, etc. If you are camping overnight in a windy area, you can use cloth to stop the rear vent from rotating.
Unless the load area of the van roof is lined, you will get rivers of condensation/rain from the metal roof during winter. You can line the roof using marine carpet:
https://www.bunnings.com.au/ideal-diy-eildon-denim-flat-marine-carpet and Selleys contact cement.
You might consider using clear plastic to seal the area between the driving compartment and the load compartment like this:
www.clarkrubber.com.au/products/242p-general-purpose-clear-plastic. Clarks should be able to cut size to suit. In summer, the air conditioner will be keeping the humans cool, not the stuff in the load space.
Finally, but not finally, you need to protect the load area lip at the rear door and the sliding door from water drips off your gear as you load it into the van. See Jerry's VW T6 van, Bondy's HiAce van and the Ford Transit van for ideas on how to accomplish this (and many of the suggestions above). I'll send you some photos by PM of load lip damage.
Suggest you allow at least 3 weeks to protect the van, allow the fish oil to dry and build your racks, etc. As we are coming into the Victorian wind doldrums of Autumn, there should be plenty of time available for your fit out.
Hope this helps.