Re: Catalina 27
My current boat is a Catalina 27 - also in the PNW. I'm assuming you're planning on sailing only in protected waters. This is a great boat for that.
Sounds like you already know some of the things to look for. The problem with the chainplates is that, like many boats, they pass through the deck. So, you can easily inspect the little bit that sticks out above the deck, and you can go below and see the part that bolts to the bulkhead, but the part that fails is the little bit where it passes through the deck - the part you can't inspect easily. With an older boat, there is a good chance they've been slowly corroding for decades and no one has ever inspected them. This isn't unique to the Catalina 27, but is a consideration.
The keel bolts issue you mentioned is something we found on our boat (a 1972). On the cabin floor there is a long board over the bilge well above the keel. Removing this board (very easy) reveals the bolts that are partially responsible for holding the keel on the boat. Water has a tendency to collect down there, so these bolts tend to rust significantly over time. Catalina recommends simply adding new bolts alongside these corroded ones, rather than removing the damaged bolts.
Ours have been badly corroded since we bought the boat, and we have not made the repair. We also have never seen any sign of the "Catalina smile", despite sailing the boat hard for several years and even running aground pretty firmly once. In other words, that keel (ours at least) is pretty solid. I've considered adding the new bolts, but the project has never made it to my short list of things to do. I don't know how much of a gamble this is, but my sense is that the boat would sail another 44 years without losing the keel. I've never heard of one coming off.
The things to look for are really just all of the things you'd look for on any older fiberglass boat. Pay particular attention to the state of the standing rigging. The fiberglass bits will pretty much last forever. These boats were over-engineered at the time, and really stand up well. But, standing rigging doesn't last forever, and repairs can be very costly. A surveyor that is well-versed in sailboats could help identify problems.
Another argument for getting a survey is that the surveyor will likely have that little device that measures moisture in the wood core of the deck. Finding a lot of moisture wouldn't necessarily be a deal-breaker, but it would affect the price.
We paid only $3500 for ours, but paid for a survey. As I recall, it was required for insurance, due to the age of the boat. So, that choice was easy.