First off, welcome to the forum my advice, and maybe I am reading the first paragraph of your post wrong, but if you plan on putting frogs in this tank at any time, they should be your primary concern and focus, not the orchid. The tank should be designed for the frogs, first and foremost, especially when darts are concerned. The reason I say that, is because, not all darts are the same. Say you build a glorious paludarium, and then decide you absolutely LOVE auratus, or a tinc species, which require floor space, you are kind of limited. They would be much happier with a full floor plan tank.
When you say 'moth orchid', I am assuming you mean a phalaenopsis species. Unless you are planning on getting a dwarf cultivar, most phals, will grow too big for an 18" tall tank. Especially when you consider how much height you lose between a false bottom and substrate. Even an 14" dwarf cultivar is going to cut it tight. The other issue with a phal and your plan is with the misting system AND a fogger, you need to watch for rotting. They like humidity, but when you hit the upper 90's, things tend to get dicey with some orchids. Also consider that orchids require excellent air circulation, especially phals, so you may need to add a fan into your design.
As far as threads to check out, hit the vivarium sub forum. There are stickies on vivarium construction that should get you on your way
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