Your best bet would be to promptly get rid of the moisture, as it will entirely determine the corrosion of not only flomweters but everything else in that system.
The moisture, if present as condensate, will also affect how well the vapour-phase flowmeters you select will work.
SO2 dissolves in water to form sulphurous acid, which is a weak reducing acid.
In the absence of water, stainless steels are fine with the liquid and vapour. , In the complete absence of oxidizing conditions and the presence of minor amounts of water, 316 SS is apparently quite resistant. 304 SS is apparently substantially less resistant when the stream is wet. If conditions are oxidizing such that traces of sulphuric acid are generated, stainless steels can be aggressively attacked by the resulting dilute acid.
