on the slab, would be advised. granted it is assuming you have good "drainage" and min frost heave already. gutters help great amount, and then making sure land around shed/barn slopes away from it.
drilling a hole and dropping a post in the ground, if not correctly compacted, could lead to water buildup, causing the slab shift more. granted it would be inside the building. but moisture, etc... will find its way to the low spot and frost heave. all the other posts (around outside) been there for years and is as compacted as it will get.
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i have same like doing in one of my sheds. "an enclosed area" the problems i have had.. are mice and ra-co ons. getting up on top and making a poo piles, along with rat droppings (not mice but rat) all it takes is one or the other and a week and it is a stinky ugly mess to clean up.
i also have had problem of high wind / rain, getting in around the metal siding and roof, letting just a little bit of water in. not much to even see dripping, but enough to cause rot. i have had to replace some metal sheeting siding roofing nails along with some silicon around the nails to cure the ingress of water.
i have also had to use some "great stuff spray foam closed cell (red can)" to fill in around the metal siding, and were 2 sheets overlap on the walls.
there is a stair case to get up on top... but it is rather ugly very steep steps. the steps take up a large amount of room. if done again... i would most likely put then between the outside wall and enclosed area... so the bottom side of the stair case was accessible inside the enclosed area. or place the stair case out into the middle of the shed.
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with above said... now that i think about it... i would most likely pull the loft area out above completely. and just put 2 walls up all the way up to the ceiling. lighting is an issue, shelving is an issue, granted more area above to heat.. but... 12 feet wide, with shelving on each side, doesn't leave you much wiggle room. exception standing a ladder up through the middle. 12' doesn't really give you much room to get a vehicle in there or like to work on it either. nor much space to actually get a medium size project going in there.
find a spot for window a/c unit, along with a dehumdifer, and some sort of heater on thermostate.
install a couple cheap windows (new construction windows = nail tab around window). one on each end of the 18' or like. so a box fan can be setup in one window and pull air through the enclosed area. windows don't have to go to the outside, just point them back into the shed.