It depends. I have not heard of the early boats (pre 1985) having osmostic blisters. The original boats were given epoxy based barrier coats. Those coatings have a limited life (10 to 20 years) and need to renewed periodically. If the blisters are in the barrier coat, then it is no big deal. Its easy to touch up the barrier coat blisters as a part of reqular maintenance, and when you at some point when you strip the bottom paint, renew the barrier coat. Its no big deal.
Also some boats had fairing material where the Brewer 12.8 vs Whitby keel were inserted into the molds. If the blistering are in a line roughly 1'' to 1'6" above the keel, then that fairing material may be breaking down, and the problem is also pretty easily dealt with as routine maintenance.
Lastly, at this point some of these boats have paint sickness and need the bottom paint stripped and renewed if you care about performance. All that said, this particular boat may indeed have osmotic blisters, in which case that is a different and more serious issue all together.
As far as I know there are a few boats listed as Brewer 12.8's in Florida, Harold Oldaks boat (which has dark blue green and is a sistership to my Dad's boat, she a lot of sea miles under her keel and has been well maintained by an extremely knowlegeable sailor), a strange boat with a live aboard oriented layout rather than the offshore oriented layout of the 12.8's and which isn't actually a 12.8 (lacks the centerboard and the high quality deck gear), and Dolphin (which actually is a Brewer 42 rather than a Brewer 12.8- lacks a centerboard, etc.).
For what it is worth, my Dad's boat is the 12.8 for sale in Annapolis.