I've welded mild steel to stainless on many applications with no problems, and I too acquired a stainless steel ( octagon) cooking chamber at one time and welded a mild steel firebox to the side of it along with mild steel legs, handles, etc. then sandblasted and painted it black. I had it for over 10 years and not one sign of cracking. I have also used stainless wire and rods welding mild steel and have never had any problems there as well. Now.... I have never had them x-rayed or anything like that, but I have a load leveler on a boat hoist that lifts three tons out of salt water on a regular basis since 2004 I built with mild steel tubing and welded with stainless rods, then blasted and epoxy coated. I think if there was a problem , it would have shown up by now.
Id disregard the fact that its stainless and base your decision on whether or not it is the right size for your needs, ...if it is and the price is right, go for it.