I'm planning to replace my fascia and soffits with Hardie materials. Hardie trim comes in a variety of dimensions and is very durable. If you also have to replace some of your soffits due to rot, you should check out their pre-vented soffits. I would guess that pvc trim would work well, too. Either way, you would want to use stainless or galvanized (hot dipped) fasteners.