Older guns aren't to be used with steel shot because they've got "soft" bbls, so shooting steel shot can damage the bore...(although I'd contest that steel shot with a good shot cup shouldn't be a problem)....I'm not certain how old the 120 is, but I wouldn't think that it's old enough to be problematic...I'd ask a local gunsmith to find out for sure, or email Winchester's customer service dept.

So, if you're planning to use this gun for Ducks/geese over water, you'll have to either buy a new gun, or use non-steel non-toxic alternative shot...Tungsten for example.

Since this is a "skeet gun", it won't be a problem.

If your gun says it's a 3" chamber (I doubt it), then 2 3/4" and 3" are acceptable, if it's 2 3/4" marked, then it'll only take 2 3/4" shells.

You won't likely WANT to shoot 3" shells for skeet, unless you're just wanting a higher shot count, I've always used 2 3/4" for the lighter recoil.

Your gun DOES have a choke, MOD does mean it is modified....It does NOT have SCREW IN chokes apparently...If you really want them, a gunsmith should be able to thread the bbl and sell you a screw in choke system, but it's putting more money into the gun than it's worth.