> no room for a forge
A farrier's portable forge is the size of an end-table. The forge does NOT generally set the size of the room; it is bench and anvil and tool space, room to swing red-hot bars without risk of burning your helper, odds and ends of scrap and projects, and room to breathe.
My father and I did some alloy casting with a heavy cast-iron hibachi and Kingsford charcoal. An hour of that did not melt a few ounces of aluminum, so he added a vacuum cleaner set to blow. Nearly set the house on fire, burned a full pot of charcoal in a couple minutes, but the aluminum did flow, sorta. The iron laddle did not soften. Conclusion: to work iron you will need a denser and maybe cheaper fuel than fluffy picnic charcoal, and a deeper bed of it than a hibachi.
I'm not gonna do the math now, but a plumber's propane torch is like 3,000BTU, a horseshoe forge should peak over 30,000BTU.
Big difference between melting solder, melting aluminum, and just softening iron. Solder is 700 and iron is 1400-2800? Heat losses rise exponentially with temps above 400.
Big difference between an ounce of pipe and a pound of iron.
Any piece of metal thicker than thinwall pipe or hotter than solder draws the heat out faster than a torch can put it in. Part of the point of the large coal bed in a forge is to keep a long length of stock hot, so the end can be brought up HOT. Another point is the fairly low air velocity; a propane torch blows-away a lot of the heat it makes.
I've bent 2"x0.125" Aluminum with a propane torch. I've failed to bend 1.25" iron pipe with a welding torch; oxy/acetylene can make a hot molten spot but not a large soft spot. Well, not with the tips and hoses an auto mechanic uses for modern car work.
My aluminum bending actually failed. The high temperature causes rapid oxidation of the metal and it fell apart. A key trick in hot iron work is controlling the oxygen (and carbon!) content of the gases around the iron. Most heating is done with slack blower, the iron deep in the middle, an oxygen-poor (and carbon-rich) atmosphere. Oxy/acetylene has such control, and a welder picks a balance for the work being done. Plumber's propane torches have no control.
Good iron-work also limits Hydrogen, because it makes iron brittle; a reason to avoid hydrocarbon fuels. I suppose that's not too important until you get to building bigger stuff. But hydrocarbons also cost more. Bad for the metal, bad for the profit; the only point is low start-up cost due to simple burner.
Metal-working is almost all about Cost Of Energy. Red dirt is almost free at the pit. Cooking the metallic scum off the dirt takes a LOT of heat. Re-heating the raw iron to adjust its carbon is more heat, reshaping pig into slab or rails or casting is a lot of heat. OK, you can get all this cheap, with scrap-iron at ~~~$100/ton. But softening an old boiler-stay into a goth earring is a LOT of heat input.
Unless you get lucky, a pound of propane is $3, which is like gasoline at $20/gallon. 20 pounds hobby-grade coal is $15, and the price goes way down from there. Yes, you can get prope cheaper in big jars. It is still a very costly fuel.
But WTH. A $5 torch is only $30. You can probably soften paperclips. You may be able to forge a few 0.75"x0.1" strap iron cabinet braces before the tank runs dry.