Remember lighting is going to be important in not making it feel like a real cave. Reflective lighting onto the ceiling would bea nice touch and good for movie viewing. Using eitherwall mountedreflective lighting or creating a shelf/soffit to hide lighting. Recessed can lights are a good idea with that low a ceiling. I would prefer drywall ceiling with false wood beams. Later work and repairs on piping is a consideration, but no different between other floors and disaster repairs should be covered by insurance anyways. Because of lighting I would stick with wood up to chair rail height and wallboard above. Wood pieces on seams was a great idea. I've seen weathered salvaged wood used and it was great. They also used sections of bowling lanes as their bar counter (good timing on a bowling alley being redone). You could also use split logs for posts ( a lot more work but neat effect ) but boxing them in with wood is a good option. Put in twice the number of recepticles that you think you will need. Run sound (surround sound) and network wiring before you add the walls. Have network outlet near TV area for future uses (direct video/dvr). Some cabinets for displays (trophies/plaques/pics/books) with lighting along one wall.