Ok so after following your sound advice I have repaired the door and it's come up looking good, if I do say so myself.
I cleaned off the waxed rust preventer to reveal minor surface rust, removed it with angle grinder and wire head. Treated the rust, repainted inside of door with rattle can.
Then I decided to cut a piece of marine ply to replace the old door trim, provide some strength and tidy it all up a bit. After some trimming with the jigsaw I wasn't happy with the final fit / flex in the ply, so went to option two checker plate.
I priced up laser cutting with me suppling the plate and it actually worked out cheaper to go with a Britpart pre-cut piece off eBay. Landed in Australia was approx. $140, which I was happy with for the convenience (4 days) to arrive.
So now I have a strong door, wheel relocated to the newly installed Mantec spare wheel carrier and the rattles have gone 
 
Check out the attached pics of the final Checker Plate install. 
Thanks again for your advice to fix, it was satisfying and not too hard.
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