I agree with others - shouldn't be hard to get a non-standard door. You can easily customize however you'd like. You just won't be able to stroll into home depot and grab one off the shelf. I would shy away from solid wood unless that's what your current door is due to the weight.
For prehung vs slab, I would recommend prehung. It makes the install go much smoother/faster & you have a perfectly sealed door that way. Just need to fit/shim into the opening. If you have a slab you have to route or chisel locks, hinges, etc onto the slab that takes more time than it's worth in my opinion