Are you sure the Ac will suck out all the co2?
A window unit or a portable dual hose AC should vent very little air out of the grow room. The only type of AC that will vent alot of air is the single hose portable units which would be a bad idea for a sealed room and window units if they are not set to recirculate.
Using Home Depot exactly like the OP and managed to get a older Telaire CO2 monitor for $130 delivered only to find out it does not have a power supply and now I have to figure out which adapter will work without frying it.
If you keep watching E-bay you can find a Telaire 8000 for around 200-250 possibly less in auctions once in a while and they are the same components that are used to make he Cap ppm controllers which go for double. Not to sure about this place but if you can get anyone to verify this Co2 controller:
http://www.hydroinnovations.com/product5.htm
is any good it might not be a bad deal.
As far as cooling goes I have been thinking of a DIY version of the Hydrogen Icebox
http://www.hydroinnovations.com/product3.htm
which looks to be basically a heater core with water running through it. If you were short on funds you could temporarily run it drain to waste until there was enough money to buy a chiller. I was personally playing with the idea of plumbing in a chest freezer with water inside it in a container that was outdoors as a poor mans chiller. Seen a guy using one to cool his aquarium and it might keep the water cool enough that exhausting cooltubes would be unnecessary. A split AC unit would be more efficient but cost alot more up front.