Two years ago, I built a large 35 x 12 ft. deck with a 12 ft. section 15 ft. wide for a screened porch. I really wanted a three season room but didn't think I could afford it at the time. My plan was to stick build the screen porch and add sliding windows at a later date.
While attending a Home show, I visited a number of companies offering pre-fab foam and aluminum panel 3 & 4 season rooms. I don't know what kind of money you were planning to spend, but I would urge you to check this option out. The prices between different companies vary as much as $3000 for the same layout and, from what I could see the same quality.
A 12 x 15 3 season room, installed, ran $5000. The job took only one day and the even ran wiring for outlets and a cieling fan at no extra charge.
It's been through a couple of tough winters with no problems.