Setting this up in a closet could have ventilation challenges to an extent, as you mentioned. A couple of options that come to mind include replacing the closet door with a screen door, or just removing the door completely. That should help increase airflow into the closet.
Are you planning to have multiple enclosures stacked up within the closet? If so, perhaps you could build them with drop down doors that are either full/partial screen or glass in the center but with screened vents within the frame that holds the glass so that there is some airflow coming in from the front. I'd imagine that would be enough, but you could probably rig up a quiet ventilation system using computer case fans to blow exhaust out from the top of the closet which should draw cooler air up from the floor of the room into the closet. That will probably take a bit of testing and can likely be done after the fact if you leave a little space for it.
As far as substrate goes, ceramic tile tends to work pretty well. You can have the tile cut into an appropriate size to be able to lift out and clean as needed. I've used that with my enclosure in the past and it was easy to use. I did find that if they are very active, the hard tile can be rough on the palms of their feet and hands. In those cases, putting down something softer like fleece over the tile can help provide some relief. You can also get some rolls of non adhesive shelf liner with a sand or stone pattern and just lay that along the bottom. Those are easy to clean and cheap to replace as needed as well.
I also suggest removing your phone number from the previous post so that bots don't scan it and cause problems for you since the pages are google searchable. At the very least, if you wish to have the number posted I would replace the numbers with words and symbols that can be understood by humans but may be tougher for a bot to parse.