I have items that have been outdoors in every weather condition for about 10 years now that still look like I just made them.
Any strong quality clay will be fine. The only thing is that the piece should not be in direct sunlight because that is what would cause the polymer clay to degrade and crumble.
The mushrooms I made for my mom's shade garden have been out there for 5 years now and do not have any kind of finish on them. They are quite large, some 5 inches, some 7 inches and were made with Premo over tightly packed aluminum foil.
This sculpture was designed for a shade garden. It was a custom order I made for the Smooshers Medieval theme challenge. It is a different design so it has a much stronger armature.