Yeah then I'd suggest getting some intermediate hybrid crosses between lowlanders and highlanders for Nepenthes. Theres some very nice ones out there. I use a 5.5 GPH RO unit which supplies all my plant water and does just fine with the mist system. The only problem is the amount of pressure you need to make the misters actually mist instead of drip. For that I use a 3/4 Horsepower industrial pump. You will probably want something with a little more kick (5-8 horsepower) since your GH is bigger then mine and it's higher. You can find a misting hose easily online, they're the ones used to cool down patios/shops. My GH is a small GH that I still have (6' 6' 7'). It's such a small GH all I had to do was get 4 giant drill bits and drill into each corner of the GH (there was cement below which used to be from an old basketball court). I'm not really very experienced with foundations myself. I think your fine with the depth part of it though... you'll need that with the high winds you receive.
For the misting part of it you all probably be best off if you attach it to the top of the inside of the greenhouse and stretch the misting hose along that 2 by 4 (or is it a 2 by 6?). That should work the best for the cooling methods since hot air rises. Can your orchids take to temperatures above 95*F? I grow a few highland orchids which don't seem to mind the high temps it as long as their temps don't get below freezing.
If not then your probably going to want to think about other cooling options. A radical Idea I just stumbled upon a while ago involved a guy with a 10000 foot square GH who buried about 100 feet of large pipe about 5 feet below the ground and sucked air through it to cool his GH. This may require some calculations and a backhoe to dig up the ground to put the pipe in. You would probably want 1' diameter pipe (drainage pipe, not the PVC kind) which can be bent in a circular-like pattern in the pit dug by the backhoe. I have some of it which is used for drainage. It's not that expensive based on what I saw the gardeners install for the drainage. There was a video on youtube of it somewhere, but I can't find it. Also, if your gonna have exhaust fans then it would be a lot better if they were closer toward the ground to pump the cool air in. This would be more effective. You will also need a vent which could go right in that white box above the entrance door. You may have to shorten the two wood triangles above that based on the dimensions of the vent, most vent systems are square. I'd recommend getting a vent with an auto-opener based on the temperatures so it opens when it's hot and closes when it's cold. The exhaust fan which will be blowing in the air would have to be somewhere closer to the ground where it could suck in colder air more efficiently. Also, you could always attach a underground pipe on the back of the fan from the "radical idea" above if time and money permit and if the calculations show a wide difference from the air entering the pipe into the ground to the air coming out of the pipe into the GH. Not a lot of people have done this "radical idea" which is why there isn't much evidence to show that it cools the air enough. Still, unless you want to buy a big industrial swamper it's probably the best way to go. The cheapo swamp coolers that are usually >250$ tend to break a lot even if they do a good job at cooling. It's too much of a risk. Theres several cooling systems above those prices if your willing to go that route, and I'm sure several forum members would be willing to show you those options if needed.
I probably got carried away :P, but I hope this helps with the cooling option. In the end you'l be saving a lot of money on the GH, I've seen high prices for custom-made GreenHouses that go in to the 10's of G's. If anything comes to mind I'll post it on here .