If you are building a small greenhouse, it would make it easier than large projects to source your building materials from the waste/salvage streams.
Old windows can be found easily
enough on craigslist or kijiji, or at transfer stations. Wooden, single-pane windows can be pieced together and screwed right on to a lumber frame, or multiple-paned windows, usually discarded because the gas seal breaks, can be stripped of their panes, which can be framed and used similarly.
One of the most important things to remember during your greenhouse build is location. You can add shade cloth and added ventilation, if you need to. Situate your greenhouse where it will get the most sun.
As to irrigation, that's almost a separate issue, except that if you wish to collect rainwater from your greenhouse roof, you have to design with that in mind, thinking about sloped areas as catchment and adding gutters and a barrel. I really like gravity-driven irrigation. It lets you water with very little
energy, or even attention, should the system be set up with drip hoses or microirrigation. All that need happen is for the water reservoir to be elevated above the watering level, and for a solar-powered valve timer with a battery backup to be employed to open and close the valve to the irrigation lines at set times.
Also, worry-free systems usually include temperature-activated vent openers and closers to ensure it doesn't get too hot.
We would love some pictures, if you're willing. But let us know how it goes, and good luck.
-CK
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein