Depending on weather, there's a non-zero chance you'll see some Hunter boots with nice dresses.
Back when I lived in suits, I had several sheath dresses with matching long jackets; that's what I would wear to a wedding like this. (It's not expected to buy something new and impractical.) Accessories can make it festive, if you like, but it's more important to be dressed respectfully (for the church and/or civil officials) and not to outshine the bride.
I would wear warm stockings and nice shoes, and a warm wrap in a flattering color, and have boots in the car.