You can also accomplish something that could be both manually adjustable or pc controlled cheaper with a pic system. I still think it's probably cheaper and less time consuming to buy an already made system and I have programming and electrical engineering experience and all the equipment to do this sort of thing. A pic system would be the cheapest way to what you want to accomplish though, imho. If you plan to go ahead get a pic starter kit that is both usb and serial. A plc system would be less flexible but may be easier in the long run. Definitely going to be a tough project.
You could always use the accessories from the big boy packages along with your pic micro controller setup.
http://www.rollitup.org/grow-room-design-setup/196997-microcontroller-auto-adjust-temp-humidity.html
Thats kinda what I was thinking but I don't have the time for this project. Lookslike someone else is going to profit from my laziness. But in the end I will profit. Stupid humans only profit.