Of course I will be using the Accumonitor purchased from you Ron!
I am plumbing in a RO system that will feed only the humidifier. The location of the closet is inside the bathroom of this area, so the bath sink is on a wall that shares a wall with the closet. The RO system will be mounted under the sink and plumb through the wall into the humidifier, which will be outfitted with a float for auto fill purposes. No need to purchase distilled water this way and was relatively cheap. 5 stage RO system with the float needed was only $160 shipped. They use these systems for reef setups, which works out perfect for this setup.
As for vapor barrier, etc. after an extensive search on the internet and questioning a couple local B&Ms how they did theirs and also the company that sold me the cedar who also builds humidors for shops, I am just going with 2 coats of BIN primer and a couple coats of mildew resistant paint, then the cedar on top. If this was a new install i.e. a new room then I probably would have went with the green board/cement board as a precaution, but everyone I have talked to says it isn't needed.
As for cooling, this closet is located in a room that I use as my office, which I keep at 70-72, so I don't think I will need any further cooling, but if the need arises then I can always run a duct into the closet as this room has its own HVAC system that only feeds 500sf. I personally don't think I will need it, but this summer will be the true test. Since it will be constantly humidified I don't think the little bit of dryness that comes with A/C air will have much affect on the room as far as dropping the humidity much.
It is a little out of my building skills that I have done before, but nothing that I don't think I can't handle. When looking at it the hardest part will be cutting the crown and baseboards at the proper angles and then the routing of the hinges for the new door. Both of which I have never done, but look fairly simple enough. I have most of the tools needed, just never needed to do this things before.
Total cost is around $2800-$3000. I went over board on some things like the RO system, crown, baseboards, etc made from cedar, but in the end I think they will finish the room off nicely. Should have completed pics up by end of the month.