ZW's are great transformers. There are a lot of guys who do an excellent job of repair/refurbishing, and some who specialize only in transformers - sounds like you got a good one!
As Lionel Train repair is my business , I see lots of positives in your new purchase: new rollers; new AC cord [a must! with 60+ y.o. items!] whistle & direction controls center on their own - this is a biggie - as it's an expensive repair if they need replacement! 0-21V AC is full power; some register at 22V but I usually chalk this up to meter sensitivity.
Looks like Santa showed up at your place early! Lots of luck and many a pleasant hour enjoying operating your trains with your new ZW!
Merry Christmas and a Happy Holiday Season!
Len Carparelli