Wow the is a lot of good information in this thread! I wish I had been able to read this 6 years ago.
When I was a new smith just getting started, I built a forge and wondered home much blower I needed. I didn't know about this site and just went ahead a bought a blower, too much. I believe it is 400 cfm and maybe more than 5 inches of water column. I quickly found out that 400 cfm was very excessive. I put a knife gate on the discharge to control the flow but the noise of the blower at full speed was very much a problem. I installed a speed control to turn down the rpms to a reasonable rate and now use the knife gate just to shut off the air.
I would guess that 100 cfm and 3 inches of water column are sufficient flow and pressure for a typical coal forge, assuming you are using a 3 inch diameter duct to the tuyere.
As far a building a blower, I guess I wouldn't go to that much trouble when there are plenty of sources around. As far as rotational speed goes, tip speed is where the noise comes from. If you exceed sonic velocity, the noise will increase dramatically. That is what I don't like about vacuums, small diameter rotors at high speed.
Good luck with your project.