I wouldn't stress about a vapour barrier in this set up.
The biggest thing is insulation. You have a few options:
A)Buy new panels - very expensive
B)find a grovery store that is getting demo'd and show up with cash/beer make a deal. Or check used building supply stores - could be expensive, could be cheap
C)Frame walls with steel stud, not wood, and use a 2lb spray foam. This is probably the best do it yourself method
D)steel framing with rigid insulation
E)Option C or D but using wood instead of steel stud.
Steel stud over would is a no brainer for me. More durable, longer life span.
A cooler is only as good as its insulation. When it comes down to insulation, its only two options in my opinion, Rigid or closed cell spray. Less insulation means more time cooling. More insulation means less time running. If its just to hang a deer every now and then, batt would work but your AC would be running for most of the time.
On the steel studs I would install a densesheild style board. I would then epoxy the floor and run your epoxy about 8-12" up the denseshield. Then install FRP panels. FRP are fiberglass reinforced panels and that will hold up in an environment like this and can be washed easy. After FRP, caulk all the joints.
Your room is large and will be dark, install a light suitable for wet spaces.
I personally wouldn't insulate the floor unless this unit was running 24/7 as a cooler. If the floor is concrete with a drain, it will naturally be cool being partially buried and not exposed to the sun. Cold air also travels down.
Lots of options. Majority of the project will be driven by budget
We converted the front 4' of our cargo trailer to have refridge when in the bush. Coolbot works like a charm.