Until you have a 100% certain diagnosis I would not replace an item this expensive. drive it till it dies or progresses to certainty. IMO

(you mentioned 111k miles...how often did you change the fluid?)

I would only buy a BMW remanned unit. Just having someone 'do a rebuild with a kit' may not be adequate- with a reman each and every out of spec part is replaced...with a local rebuild all you get are the parts in the kit- anything else is worn, tough luck- back in it goes,,,. And BMW parts have a 2yr warranty. Lots of online BMW dealerships. And your local one will probably match the price if you have a chat with the parts manager.