A smokehouse is not designed to "hot cook" food like you asked about. The store bought models are basically dual purpose. They can run at lower temps for smoking and higher temps for cooking. What is in the photo you posted is a smokehouse, not a "cooker". You could very likely get low 200-230's out of a design like that if the food chamber is insulated and that will "cook" most smoked foods just fine. But you are not going to get 275-350* out of that (I would not even try it in a wood framed smokehouse). But that design is mainly for lower temp smoking and not cooking.
I don't see why you can't use the salvaged barn metal, with a couple of cautions. The most common caution you will hear is don't use galvanized metal. In a smoke house that is less of an issue as the temps involved are no where near high enough for the zinc and other metal fumes to be an issue. Also the interior will quickly accumulate a coating of smoke further sealing the metal. "Metal fume fever" is an issue with galvanized and certain other metals exposed to much higher temps than you will ever have in a smoke house. The only place you might see those temps in a design like in the photo would be in the firebox, especially where the hots coals and burning smoke wood are in contact with the metal. But even that being said, once you burn off the coating, it's gone and you have the bare metal beneath.
If you look at the interior of all of the commercial smokers like the MES, Bradley, etc... they are all painted or powder coated. Never hear anyone voicing a concern about the paint/coating on those do you? And again, after a break in smoke, the whole interior should have a nice layer of "smoke" coating everything anyway (and it will continue to build up with use). You will even get that smoke buildup on a wood lined smokehouse.
As to pressure treated, you can still use that, but not in areas where it will be either in contact with the food on say as a lining of the food chamber or smoke path. PT framing underneath the interior lining, or PT exterior cladding should not be an issue as those areas should never see temps high enough to cause a problem and there should be no path for any off gassed fumes into the food chamber.
One other thought as to a barrel stove for a firebox. It will work, but the design in the photo has a thick brick & concrete firebox and flue to the smoke chamber. That design (as in the photo) will hold in a lot more heat than the thin metal of a barrel stove and a metal stovepipe. Again the more heat you can hold, the easier it is to maintain a steady temp and smoke. That design (the photo) should be very stable once at temp and running even. An uninsulated metal fire box and pipe will require a lot more tending, burn more fuel, and likely have higher temp swings, but it is workable. Some designs just work better than others (which is also why you see insulated fireboxes in the higher end big stick burners and other smokers - it's all about retaining heat and keeping it even and steady over time)
