I have been considering a similar project, except mine is a single door with 2 sidelites. I've just about decided to just go with a fiberglass pre-hung unit, instead, I think. I've never built a door, and it seems a bit overwhelming. However, my research has indicated that the best method is indeed stave core construction. There are at least a couple of companies out there who will build the stave-core rails and stiles to you specs, complete with veneer. Then you cut the parts to length, mill the profiles and assemble. If you google " stave core door parts" , you'll see at least a couple suppliers.