First is the detail of construction; asbestos, knob & tube wiring, lack of insulation, aluminum wiring, etc.

Is there no heat to the room and/or proper insulation?  

No insulation, outrageous gas bill.

I have also picked up afew lots with scrap mobiles on them and lots of junk that I got because I had heard about the pig stys in a nicer area these were crap at lake eufaulaI contacted lots of owners and in some areas they won't allow mobiles anymore and neighbor were happy I bought Let junk collector I know take anything he wanted and he took everything even rotten lumber I did have to pick up some insulation and afew other odds and ends and had it mowed trees trimmed

Site-built homes have numerous areas in which to place utilities, and they usually don’t close the only escape route for occupants, as happens many times in mobile homes.”In addition, pre-1976 mobile homes had less insulation in walls, ceilings and floors, uninsulated air ducts and no vapor barriers in their roof cavities, making them an increased risk of fire hazards.

You will have Florida Building and Energy Code standards to adhere to which will include getting a set of Energy Calculations, making sure existing cooling systems can handle the additional square footage or replacing/upgrading if necessary, and making sure the newly enclosed space is properly insulated based on current energy code standards. 

Brick has insulation issues though so there’s always pros and cons to most things.

If you do something very basic in the basement like adding a bathroom and framing, insulating, drywall, plenty of outlets and lighting in the rest with basic trim and flooring you should be able to come in under $10,000.

Because as the ones in the best position to see the demand cliff coming, it makes perfect sense to pivot your business to those boutique properties unique to your channel and luxury rentals better insulated from the constraints of average traveler counting more dollars in 2023 than they were in 2022.

In your area 2x4's would be worth more mostly because more insulation can be stuffed in there.