December- in a van in 45 degrees will be fun. There is No Water & Power available. Toilets are long drops.
Speak to the Chapman Valley Shire.
http://www.chapmanvalley.wa.gov.au/places-stayAs for running a business on the beach, I'd check that out first.
Coronations is fine if you are intermediate - as a beginner, there isn't any shallow water, the launching is sketchy, can be weedy- and there might be regulations you would need to adhere too.
Its a fun place to kite. And a very raw part of Australia. Head up north to the Pink lakes and get some great photos.
By the way, there is your flat water in the background.