Might be a good idea to visit any high quality B&M you have in the area and have a discussion with the owner if possible. Ask some advice for construction of humidors on a larger scale. If he is knowledgeable, he may be able to give some advice someone may not think of. I have one person in a discord that has a home built walk in. I'll see if I can get some tips from him next time I see him around.

For one, what type of dry wall is preinstalled in the room? Is it mold resistant? If not, you're really going to have to figure out a good way to completely seal the interior from being able to push too much constant moisture into the drywall localized in there behind everything.
It also depends on what type of shelving you want. I wouldn't really think you need the ENTIRE room made of spanish cedar. You could easily just do shelves made of it. Probably a somewhat easier and cheaper route, also, would be to moisture seal the room with whatever insulation you choose and some moisture proof/resistant paneling for appearance sake. Then do your shelving, and just store in boxes on the shelving systems. You could also get some of the premade shelving made for wine coolers to store singles/bundles or other non-boxed purchases.

The only reason I suggestion this route is the B&M most local to me has their humidor set up this way. All glass sided, and pretty free of spanish cedar - except for the cigar boxes all within. It isn't a large store, nor a large humidor. It may actually be pretty close to the size you've shown dimensions for.Even with some loss, a humidifer of this size should cover such a space, and it - in terms of what you're looking to do - doesn't break the bank.
https://www.homedepot.com/p/AIRCARE-...1407/205446600

That part is actually probably one of the easiest solutions to what you're looking to do. Also! Be sure to use all weather or marine grade metal in any instances you can. Screws, nails. Coated racks/shelving supports etc. Keeps them from rusting and bleeding into the wood. As for the electric.. I think mostly just have the sockets checked and make sure they are grounded. The humidity is higher, yes..but not so high that I think it will cause a real problem? There are plenty of places that run humidity in the 70+ range on a regular basis parts of the year. Our sockets have no problems with that relative humidity...I'd invest in a pretty good quality surge protector, though, to plug in whatever humidity system you do end up with... I wouldn't completely "seal off" the door. Unless you are going to open it daily to allow some air exchange, you want some fresh air able to circulate in and out. The humidifer should be able to make up for this without any problem caused to the cigars stored.