For indoor sunroom furniture you choose can match all the above requirements, here are some options for you. One option is Wicker furniture: rattan indoor sunroom furniture can easily withstand most of the elements of which it is exposed. it is made of a natural material that is lightweight. However, it does require regular cleaning if the cold and rain exposed as it can develop mold easily in these conditions. If you think you can be committed to regular cleaning, you can consider for your sunroom. Else you can go to the other options listed below.
Wrought iron indoor sunroom furniture: Wrought iron is a great option for a sunroom, because this can definitely support all elements of nature. It will not mold or fade in the sunlight. To protect from oxidation can use a rust proof paint on the furniture or covered with PVC. Wrought iron furniture is very elegant, and can be painted to a color of your choice. So if you want the room to look casual, you can be painted in lighter shades, and if you want the formal, you can paint in darker shades or black, because usually painted.