I have one of the M1bs, bought one of the last new ones from Eastside trains a few years after everyplace else sold out.
Its a great loco, a "big presence" with that tender but doesn't require 072.
One of Lionels best Legacy sound packages for sure with 2 full sized fatboys in that big tender, and by far Lionels best version of a "Pennsy " whistle.
I rarely see one for sale but they do come up now and then. I was late to the "scale bandwagon" when I bought mine. I believe it was mentioned on the forum at some point that these weren't that popular when they were first offered and many sold for blowout prices.
I guess folks have grown to like them. It would certainly be one of the last ones I'd let go.
Having said that, the new M1a should be pretty much the same aside from the pilot, and it comes with whistle steam.