When you are making a sword you only want to heat as much of the blade as you can work before it gets cold, generally only a couple of inches. Heating more than you can work just leasts to decarburization, grain growth and scale losses. So unless you are using a powerhammer the forge you *need* is pretty small. Except for heat treat when everything needs to be hot and at the same temp---I sometimes dig a trench forge for that rather than use an overly large forge all the time.
Gas forges can weld *IF* built for it---Hrisoulas (author of "The Pattern Welded Blade"), does his welding in an aspirated propane forge and at several thousand feet of elevation too! You would probably not want to use your welding forge for basic forging so you are talking about 2 forges, (propane forges tend to be much more specialized than a coal forge so unless you are only doing a limited ammount of stuff it's better to have a range of them built as needed.)
Remember that just because you can't see the propane exhaust doesn't make it non-toxic!
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Thomas
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