There are couple of wood species or material options for the lower section of siding. The first is more traditional; a painted rot resistant wood like western red cedar or painted mahogany. This should be primed on all side prior to installation. The next option is an newer materials called Azek. It is plastic essentially, and does not require paint, and actually looks pretty nice when installed properly. The most important thing with either material is the waterproofing at the top. Use copper of anodized aluminum, the latter will allow color matching. The metal flashing should lap over the top of the lower trim board, and up under the upper siding. You can probaly google this detail as well for more specifics.

Here we applied fiber cement sheets to the entire wall surface. Then added 1x6 miratec boards at 12" on center to create the " board and board" pattern.

We are considering closing in a large screened in porch, adding heat and air. How do you cover the underside after all of the work is completed? We live in the south with lots of bugs, critters, etc.