[Advice Requested] Convert detached "cabana" to enclosed room

This detached porch/cabana (as well the the hot tub we never use/will be selling) was pre-existing when we bought our home.

We would like to convert it to a fully enclosed building with heat/AC and I am seeking some advice.

It is 16ft x 16ft and is supported by (6) 4x4 posts (as well as some additional 6x6 supports underneath the hot tub). The interior walls are 8ft from floor to edge of roof peak. Existing studs are floor to ceiling and are 24" OC.

There is existing 110V and 220V electrical (dedicated circuits).

Our plans are to put a 36" entry door in place of the screen door, 2 windows each on the left and right sides (facing the entry door) finish the siding up to the roof line and fully insulate/sheetrock the interior walls/ceiling and install a Mr Slim-type heat/AC unit.

My questions:

1/ Do any of you see any issues with this from a structural or support standpoint?

2/ How should I handle the floor? If I pull up the existing floor planks and put down a plywood/OSB subfloor, the underside of it would be exposed to the ground/outside. I could technically put the plywood on top of the existing planks but that still doesn't solve my problem.

3/ What should be used to close off the underside(s) of the structure from the outside? It is basically a dirt/crawlspace underneath and understand there would have to be an "access panel" to be to code but assume it should be closed off somehow to keep critters out.

I'm sure I will think of other questions, but this would be a good start for some of you that know more than I do to see flaws in this plan.

I would like to do this "correctly" and mostly to code if at all possible. I am considering getting a building permit so that I have the building inspector as somewhat of a resource to ask certain questions and know that I'll be doing it how the county wants to see it.

We live outside of Atlanta (northwest), so weather is not a huge problem but we do have periods of freezing temperatures (20s-30s) in the winter.

Thanks in advance!

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