Walnut plywood installed by an experienced custom trim carpenter. If you don't have contacts, be sure to see other work they've done. You want someone who has worked on libraries, custom bars and the like.

A good place to find contacts would be the supplier of the panelling. This isn't a frequently used material due to the high cost, so they would have a good idea who orders it frequently and has experience installing it.

What exactly are you talking about when you say walnut paneling, can you find a picture of what you want? If you are talking about a true wainscoting then a cabinet maker is probably your best bet.

Defiantly call a bank. First.

Then decide if you like the mix of veneer recessed panel and solid stiles and rails that would be the cheap way to go. I don't like the clash of graining patterns. But solid all the way around a room will cost as much as a car. A more modern take would be veneer panels separated with something like stainless Schulter strips. But modern my not be your thing.

Post a picture of your proposed doors. Walzcraft makes doors and could match the door style or compliment it with something for the walls. They can supply finished or not. They only sell wholesale. Walzcraft But you can have any style or pattern you want with different wood grades.