Floor to ceiling bookshelf design: anything missing? okay to use MDF?
Done! Ended up building the base and two verticals with treated 2x4s, and regular 2x4s for cross-support around the fireplace.  Then sandwiched-in the verticals with MDF.
As you can see, I also used big L-brackets to support the long horizontals.  Likely a bit overboard there, but I think they'll look okay once it's filled. Was hoping for a long, open spans aesthetic instead of putting extra verticals in.
The MDF looks nice for now. It does have little dings and nicks. Went with a rustic looking wood pattern, so it doesn't seem to affect the overall look. So far. We'll see how well it holds up over the years.  It was certainly nice to work with, and saved a lot of painting. I think I'd use it again if I were to make another.
I haven't put the baseboard across the bottom yet. Haven't decided if I'm going to use the white baseboard that will be in the room, or more of this wood-looking mdf 3.5&quot; pieces like the verticals.