Looks like you have already given a lot of thought to this, being the 6th darkroom and all. I finished my current darkroom about 2 years ago and am always impressed with how well it was designed Of course, it took me most of a year to plan it out. I went the traditional route with drywall on the walls and ceiling, and ceramic tiles on the floor. I installed a drain in the middle of the floor for those spills that occassionally happen. Regarding electricity, I have seperate 20 amp circuts for the dry and wet side, and a 15A lights circuit, all GFI protected. The panel for the darkroom is outside the room to prevent problems with humidity. What I didn't plan for in advance was the need for a 220V circuit. I installed this just a few months ago to accomodate the L184. The GFI breaker for this last circuit cost me a small fortune! Another issue was light entering the darkroom. I could not completely resolve the light leaks so I added a curtain on the inside of the darkroom that completely envelops the door, floor to ceiling (black facing the door white facing in). I use the curtain when handling film but not for paper.
My darkroom is a little smaller than yours will be (9x10 feet) but I manage to fit a JOBO ATL2000, a 5 foot sink, a small film/paper fridge, a Durst L1200 and a Durst L184, a dry mounting press, a film drying cabinet, RC paper dryer, space for air drying prints up to 20x24, and of course enough drawer and cabinet space for all the rest. I attached some quick pics so you get a sense of the layout. Would be great if you post some pics of yours when it's all done....
Good luck and enjoy!