Agree with both of you. I'm going to make sure the masons stop any actual leaks from the outside, and I'm not going to go nuts trying to make the poly airtight. I did a first pass rounding up debris off the floor down there and I'll do an actual broom-sweep before I lay down poly.
Any thoughts on sprayfoam insulation vs. blanket insulation on the foundation walls? Sprayfoam seems superior, but if there's significant surface prep involved with that, the low-tech blanket insulation might win out.