If the original regulator brushes were that worn then other components will have excess wear too. You mentioned the slip rings have wear so you're going to see false readings if there is poor or uneven contact.
Personally, I would have your existing alternator completely refurbished, Google services in your area, and go from there. They will also test it, prior to refurbishment, to make sure everything else is good.
Ohh...and your existing alternator is very well made so don't swap it out for an inferior product. It lasted 300k so try get another 300k out of it
