There are differing views on osmotic blisters. They are quite common on boats, some kinds more than others. Some people think that they are more a cosmetic issue, especially if they are small fingernail size rather than one inch size, and if they have mainly occurred in the gelcoat and not penetrated the epoxy hull.
If you are planning to keep the boat a long time, I would want to repair blisters rather than ignore them. Repair involves grinding each blister a bit, drying out the hull with heaters or hot weather over a longer time, filling the blister holes with epoxy, usually adding an epoxy barrier coat made for this purpose, and covering that with two coats of bottom paint. Alternatively instead of grinding each blister, the gelcoat can be peeled with a machine, re-faired as necessary, epoxy coatings, bottom paint.
A boat yard can charge many thousands of dollars for this work, so get a few estimates if possible. If you can do the work yourself, it's messy but can save you alot of money.
Either way, a boat with blisters should allow you to negotiate a lower purchase price.
Let us know what you decide, and good luck!
Frank