What about insulation? Wool/Fiber vs firebrick vs cement/concrete?
That depends on what the majority of forging you're going to do.
My forge is a 16" dia pipe with 2" of refractory cast into it. But the first 8 years of my blacksmithing was doing ornamental stuff and sculptures, so I wanted a large forge to hole large, odd shaped pieces. What's bad about it is that it would take 30-45 minutes to get the forge up to welding heats. What's good about that much castable, is that it's a big heat sink, so once up to heat, it would stay there even when putting a large chunk of cold steel into it. Also, once up to heat, I could turn the propane down to ~1-2# pressure and still maintain the heat.
I also recently added a PID controller and the large thermal mass keeps a very steady temperature.
In the past few years, however, I have been doing mainly kitchen knives, and this is way overkill, so I recently modified the forge, making a smaller chamber with ceramic kiln shelving for the floor, 1/2" M-board for the walls and ceiling and kaowool filling the gaps between the original, round cast chamber and the new box-shaped forge. On the plus side, the last time I timed it, it was only 15 minutes to get to 2100F with me working on stuff after 10 minutes. The negative is that the M-board reflects heat more than absorbing it like the castable refractory, and so there's a significant temperature drop (~ 250 degrees F) when I put a cold billet in. Also, my PID switches the propane to high more frequently with this set up than before.
If I were doing only knives, I'd probably make a forge with ~6" dia opening, and with 1-2" of Kaowool with 1/2" castable over that and then coat the entire thing with ITC-100. That's how the forges in our mentoring center are made, IIRC.
If the forge is in a shop with access to electricity, you might want to look into a ribbon burner. I got mine from Pine Ridge Burners, but there are plans out there to make your own using crayons if you are so inclined. They are MUCH quieter, but you need power for the blower.
I hope that helps.
