I've been looking into a greenhouse too here's my equiptment list I've made after reading a bit:
-Heater , depening upon your climate and GH size this will probably be your biggest expense. It's said that the traditional gas and oil fired heaters can give off fumes which are harmful to plants. So that leaves electric heaters which are "safe" but expensive. I have found some hot water heating units online but I don't know if they'd work for me this far north in the dead of our minnesota winter.
-shutters I'd get motorized and hook them up to a humidistat and temp guage.
-humdifying device either a fogger/ultrasonic humidifier, misting system, etc
-back up lighting for winter's lower light
Then of course there's miscellaneous things like benches, taps to run your hose or water purifyer, electrical power hookup, etc. And probably a lot of other things I haven't even thought of yet!
It would be awesome though! If you live in a warm area you could probably grow highlanders year round in a cold frame(big wooden or steel frame GH covered in plastic) with only the watering and air circulation equiptment and be the envy of all!
I know there are a few folks here who have greenhouses-hopefully they'll be willing to share their input (for both our bennefits).
If you have a "Half Price Books and Records" chain store in your area they have bought the remaining copies of "The Complete Greenhouse" and divided them between all their stores so you might get a copy of that. I did but since Nepenthes and things require a bit different care than say tomatos and carrots this is pretty much a good starter (i.e. for needed equiptment, what shape is the best, etc but it doesn't cover all that I would want to know for my purposes.
In addition to the frame, covering, electric and water systems...
Heating system; depends on your area how big of a heater it needs and how warm you want to run it. Gas/oil/propane
Venting system; motorized exhaust fans and either motorized or non motorized intake shutters.
Cooling/humidifying system; misters or wetpad
Water purifying system (included since we are talking CPs); RO or Dionized depending if you have to pay a utility company for water usage or not. Rainwater if you can rig a system to collect, strain, and hold it.
All assorted parts and pieces for the above systems
Miscellaneous items etc...
Gravel for the GH floor
Items that are 'optional'
Backup generator or heater that does not need electric!
Warning system if temperature is too high/low or electric power goes out
Automatic watering system
Generally speaking it is cheaper and easier to build a grow room in a cellar or something instead of a Greenhouse. Greenhouses are nice because they can be big! Gas/propane and oil heaters are perfectly fine to use but they do need to be vented properly.