My husband has built three from scratch. It is alot cheaper! He got some stuff at the dump for free, some at yard sales and some used commercial greenhouse plastic from a nursery that had just removed it from one of their long greenhouses because it had a tear in it. We have enough plastic for life! He did buy the large poles for the frame of the greenhouse. He made his own doors at first. Now he uses old screen doors from a yard sale. He also bought alot of old windows at the same sale to make the next greenhouse with instead of using plastic. Not pretty, but the plants or I don't care. We have also used folding chair webbing to staple the plastic on with when we ran out of the black cloth nursery tape. Another yard sale/thrift store item. Much cheaper and works just as well

In general, most of "cheap" kits are not that sturdy. And often will require some additional bracing and extra work holding the glazing in. There may be exceptions, but I have not heard anyone saying that any of the inexpensive kits don't fit this.

Whether that is OK depends a lot on your locale, wind and snow loads, and how you plan to use your GH. If you aren't planning on having plants in there all year that can't take an occasional freeze if a panel blows out, and you don't have extreme weather to deal with, one of those kits may work well for you.

You might also check out (and post the same q) over on the Greenhouse and Garden Structures forum too.

I built. See my profile for pictures. I was able to build taller and add a heat sink, but buying would have been faster, cheaper, and fewer headaches.