Sorry for taking so long to get back to this thread! I've had a student in the shop for the past few days, running daylight till dark.
OK, I'm gona be really frank on my opinions about forges.....first and foremost, the simpler the design, and the fewer devices and fittings you use....the better! I've spent a LOT of hours on the phone with some of these folks who insisted on building a PID controlled forge or something similar with lots of fittings and devices installed....the more you overtake the plumbing, the easier it is to stop up the drain!
Next, for an all purpose forge, I think that 8-10" pipe is the way to go. Length wise, if you're using a single burner, about the max length you can effectively heat is 15-17".
Insulation/refractory: Whichever you choose, you want the highest heat rating you can get for the chosen material. Here is where I will step on some toes....if you choose ceramic blanket, it should be 1" thick X #8 density.....NOT 2", nor two layers of 1". You can do it if you want, but your just wasting your money. 2" of ceramic fiber blanket requires a longer heat up time, and uses more fuel to maintain a given temp within the same interior dimensions.
Personally, I choose NOT to use the "safe wool" because the temp rating is so low compared to standard ceramic blankets.
Next, interior coatings...this is where it's going to be a "give and take" situation. Generally the "coatings" that resist flux the best, suffer in the area of insulation qualities...and those that provide the best insulation qualities suffer in flux resistance. I'm always experimenting to find the "magic bullet", but so far have not found it. Since I do this for a living, it's more important for me to have the heat when I need it, and durability takes a back seat to that. With that in mind, I use a slurry of ITC-100 to coat down the interior of my forges. ITC is not very flux resistant, but I can pull 500-1000F extra with it, versus without.
An idea that I'm going to try the next time I reline my welding forge is a slurry of Mizzou (which has very high flux resistance) over the wool, and once that cures, try going over it with the ITC-100 slurry.
If you're wanting one forge for all around use, I think a forced air burner is the only way to go. You can always turn down the air/fuel for general forging, and you will have the "umph" to weld when needed. Venturi burners simply cannot match the heat output and the economic fuel consumption of blown burners.
In the end it, you want something that first and foremost fits your needs. But you also need to consider reliability and ease of care. The burner image that I posted earlier in this thread has been in service, basically unchanged for over 10 years. The ONLY maintenance it has ever required was changing a burnt out blower, and pulling the gas line once, to remove a tiny particle from the orifice (it no longer has an orifice..the needle valve accomplishes the same function).