Well at least you have an idea on what your planning on, so I hope my greenhouse talk will be ok.
First decide what covering you want, I would go with poly myself, double layered inflation if possible. Decide what frame you want, I chose the galvanized hoops (well they were given to me.)
Get a reliable heater!!!!!! Mine broke down and all my plants froze...luckily I didn't have an extensive collection and most of the plants did come back better than they were growing before!
Get a shade cloth, preferably around 50 to 60% density. You can just shade part of the greenhouse too, so you can grow sun loving CPs on the other side of it. Nepenthes for example could go on the shady end.
Insulate the foundation. Make sure you get all the nooks and crannys sealed up with silicone and/or Great stuff, that foam in a can. You want a greenhouse to be as air tight as possible so it will retain humidity and temperature (for the winter months). Also if you are going to be utilizing an evaporative coller it needs to be extra airtight, all air must be going through the pads for maximum humidity and cooling output.
Get an exhaust fan big enough for the greenhouse. I personally have a big 3 foot diameter fan that has a CFM of 11,200 I belive. Of course ou may certinaly not need a fan that big but be sure it meets of exceeds the CFM of your greenhouse, it's better to havea fan a big bigger than a bit smaller. You want the air to exchange in the greenhouse about every 1-2 mintues so on a hot day it will be nice and breezy in there, not hot and unbearable.
Ok, that about covers all the neccesities. Here's a photo of my private greenhouse.
Any questions just ask.
Good luck!