If you plan on painting the fence, use treated timber posts concreted in. For the rails ............. I would go for uPVC. Easily fixed to the posts. It's very flexible, you might need extra posts on the tight curves to keep it from flopping around.
Then fix vertical boarding, or whatever, to the rails.
The uPVC can be painted along with the rest.
If you go for stained timber, then maybe use brown or black uPVC.......... bit of a compromise, I know, but could be made to look good.