Heat retaining power is better than glass or plastic film coverings. In warm climates, a fiberglass greenhouse requires less cooling. Light transmission is good. Insulated panels of rigid plastic are available. Fiberglass can be put on a straight surface or a curved one.
Some rigid plastics are UV resistant.
Foundations for a greenhouse can be solid concrete or a more lightweight foundation. Many use railroad ties to attach their greenhouse frame to. Others just stake the frame to the ground. Just be sure and secure it well for the place you live. Again it all goes back to how much money you can spend.