I put a "storage building" in my back yard...I heat mine with electric baseboard heat. I use an efficient 110VAC window AC. I put the AC unit UNDER the layout to keep from destroying the scenery views...if that does not bother you, then put it where you want. I only run my AC or heat about 90-120 minutes before I plan to work on the railroad. We hit 113 in the summer and we get some zero in the winter. I might turn the HVAC on a little earlier on those days...but 30-90, I can get it comfortable enough for an old fat man to not smell bacon all the time I am in the train building. (smile)
To be honest, I went overboard on heat and AC. I ran a 220VAC circuit into the building. When I built the building all efficient window AC were 220. When mine rolled over and took a permanent vacation, I went to the big box store and discovered that window AC has improved significantly in the last 20 years. Same BTU, lower utility bill due to higher efficiency and it runs on 110. I still use the 220 VAC for my baseboard heat...but I installed two 8ft panels and honestly only use one...on the side of the room where I am working...by the time I get around to the "other side" (16x20 exterior building) it has warmed up enough to be comfy.
My layout is a "duck under". By using a duck under you will have longer runs for the oval. If mobility is an issue, I know one guy who put a rolling office chair at the duck under point and tied a clothesline rope to the arm of the chair and a bracket on the wall and on the layout front both, so regardless of where the chair was, you could pull it to you to sit in and wheel under the layout.
I think the suggestion for metal studs on 16 inch centers makes a lot of sense in Florida. They have some termites that scare the pit bulls tied to the porch with logging chains. Or so I'm told...(smile) The simple insulation would be batten. I would consider "covering" the outside of the stud wall and insulation with peg board so you can hang tools and such on the wall. Since those walls won't actually be exposed to the elements, you should be able to hold the "conditioned" climate inside the room.
Most of that should be able to be salvaged if you decide to move to a "storage building" in your back yard in the future...I'm just saying....drop a metal shipping container in the back yard. Take the two doors off and install a stud wall with a window (AC) and door. No termite issues. Drive some steel I-beams into the ground (tractor with back hoe) and weld the I-beam to the shipping container. Paint it to match the house...be sure to put a full 36 inch door for ease of carrying supplies and building material into the storage container. With that "tie down" you should exceed any requirements regarding debris in hurricane weather...maybe. (I am not a civil engineer or architect and did not sleep in a motel last night, you are on your own if you elect to follow that suggestion.)
What ever you decide...enjoy the move to Florida.
