I think that if you buy a boat, you directly ask the surveyor his
opinion on whether or not the blisters are a structural issue.

I have owned two boats with blisters. The first I repaired by drying
for 3 months and then grinding them out and filling with epoxy and then
several epoxy barrier coats (I know, not the same as a gelcoat peel)
and when it looked beautiful, it went back in the water and all my work
was hidden. A total waste of time as none of the blisters was into the
lam. The inside of the boat showed no problems at all.
My current boat has some blisters and my strategy is to repair any that
look bad and ignore the others. I had a surveyor look over the boat
(not a real survey) for insurance purposes recently and he stated "No
obvious blisters" even though I had seen them . I think that if a boat
is over 20 yrs old and the blisters have not becom a structural prob,
they arent likely to ever be.