I have a smaller forge, it is 4" round by 14" long. It got to welding temps as a venturi, but I changed to forced air for controllability. I too was looking for a cost-effective fan. I am using a blower like this and am very happy with it. It gets to welding temps quickly and makes the forge...
Ha, it must be a 4". I remember doing the math when I built it. I wanted more pressure but as you know what you got is what you got unless you spend more. I do have it turned up as high as it goes.
The billet is sized to a little under 3/4" using flat dies with spacing stops on a press. The twister uses sockets made from 3/4 " socket extensions, one of which is turned down to fit in the drill chuck.
At Automation Direct, the correct gear reduction (60:1) is $261 and the 1 hp motor (1750rpm) is $253. I'm pretty sure I could source the rest of the parts under $500.