I think SS mig welding might have the highest toxicity air borne. Your situation is on the smokey side, which is useful, in that at least you will see the effectiveness of your extractor solution.

I misspelled my first reply, I intended to mention "variable" speed.

Sorry, being from Florida, i didn't consider makeup air issues, you could if you wanted to get fancy, and save btu's in the winter, set up a two speed fan, that would trigger the higher fan speed off the welding arc. Not sure where your fan will be situated, but I hate constant fan noise, enough so on my Miller Synrowave 500 machine, I installed a low speed fan switch, and added some sound absorption material on the wall behind the machine, I like my hearing as much as my lungs.