I'm no expert on greenhouses but I'm guessing the materials you choose will depend on location of the greenhouse (wind/rain/snow) and how much you intend to use it. How many plants do you have, and are you wanting the greenhouse for future plants? Corregated panels work well but if you're in a high wind area be prepared for damage and noise. For longevity, I would consider glass panels. You could check salvage businesses for glass panels or glass doors to use as your walls and you'll also need some type of window/vent to open during warm winter days.

As for the GH also being a catio, I'm not sure how much the cats will use it during the months you have clear paneling up. Clear, plastic paneling isn't really that 'clear' and if cats can't see out one would think they'd have no interest in spending any time out there. You may want to consider 2 structures.....one attached to the house for the cats and a separate building for the greenhouse.

I have a catio on my deck that attaches to a cat door via a wire tunnel. The catio is a 6x10 dog kennel with reinforced mesh for the roof. Fortunately we have a covered deck so we didn't have to build a solid roof. I'm in the Portland area and my cats use the kennel year round.......avoiding it only during the windy, stormy days. The insects, crows, other birds, deer, etc keep the young cat entertained while the senior cat sleeps. :).

I guess you don't want to use the multiwall polycarbonate for the walls because you want a clear glass like view. If you have a mild winter, as stated, I would build a removable wood frame and insert this clear view vinyl covering into the frame. That way you would just remove the entire frame with covering every year and store. This is a truly glass like look from this material. Regular 6 mil, 4 year greenhouse polyfilm is not completely clear.