I have a steel white Tecrostar mezzanine floor kit in my downtown loft apartment that I bought new from the manufacturer in Spain 2 years ago
10’ x 14’ (140 sf) with a spiral staircase and two guardrails.
It’s very sturdy and high quality.
It’s not free standing, it needs to be bolted to at least two supporting walls. Although there is a stability kit that can be purchased from the manufacturer to make it freestanding as well. They also have flooring materials that allow it to be an outdoor structure with maintenance instructions.
It requires at least two strong people to disassemble and move with a big truck and reassemble. I have the manual with diagrams and spare parts and all original receipts.
It’s currently assembled in a 16’ vertical space but the height of the floor and staircase are adjustable. The area size of the floor is also adjustable but you would need to cut new floor boards if you changed that.
The page for the main base structure is here but the stairs and railings are extra and shipping cost a ton as well:
https://www.tecrostar.com/mezzanine-floor-medium/structures-