I specify that structural materials that are welded to the pressure containing envelop shall be fabricated of materials that essentially the same as the head or shell to which they are attached. Consequently, I'd use SA240 Grade 304 plate material.
Regardless, you would want to make sure that whatever material you used, that the skirt material was suitable for the MDMT of the vessel for at least a distance equal to that which it could be affected by the vessel temperature. Figuring out this distance could be a bit of a problem so it's probably just easier to use 304 plate for the whole thing.
I have seen SS vessels with a ring of the same material welded to the bottom head, to form the top of the skirt, and CS plate used below that. Depends on the size, number, as to whether that's economical or not, and whether long term corrosion protection is an issue.
low temp a333 is the worst case. you could use a333 for a foot or two then switch to a105 or grade B pipe for pipe diameter vessels if you believe the insulation is poor.
Petrochemical and Oil & Gas has always had a preference for skirt designs. If you look at the Process Industry Practices (PIP) they allow leg designs under only certain limited situations if I recall correctly.
PIP specify that the support should be SS for the portion of 2*(Rt)^0.5 but not less than 300 mm (12"). The lower portion can then be CS or low-alloy steel.
