I feel like this could have been written by me, seeing our conditions are more or less similar - some leftovers left by previous house owners as the source materials, possibly not-so-clear idea of what and how to build, and an unreliable result.
Despite what I wrote in the reply to you on my post, I might still set up some sort of a shelter for the plants this summer. I do remember seeing some pretty sturdy pieces of timber in the pile I have, and I can get more from my parents if needed. The only thing I'm missing entirely is glass or plastic for the roof and walls, but that I may be able to buy somewhat cheap if I only use a film rather than solid sheets.
How's your greenhouse project advancing? (Found the newer post, never mind...) I'm probably going to invest in a commercial one as well once I'm done with the more urgent purchases, so I'm eager to hear of your experience if you buy and build one.
I do recommend a commercial one, unless you actually have a lot of good quality materials and/or you're really skilled craftsman. I'm not one and I'm sure someone else could have put those materials in better use.
However with commercial ones you usually only need to follow the instructions and you'll get it done. Usually with help of a friend - this is what I really recommend, don't try putting one together alone.