Sooner or Later (a boat)???
The BUC book list a range of values for individual models by year and manufacturer as well as adustments for larger optional gear. It does not show buyer preferences. For example on some boats that less than 30 or 32 feet, having wheel steering in some performance models decreases resale value while on other models it increases resale value. That may or may not show up in the BUC Book. BUC Book includes adjustments for region and so on.
It can be way off on rarer models that are sold in small qualities but is pretty accurate for boats like the Catalina 27 which are sold in such large numbers that there is a good experience record for the boats.
The actual data is gathered from brokers and dealers so the value of a boat for a private sale should be below the BUC book values.
As to the details of the purchase at hand, you need to have your own purchase survey done by an independent surveyor. An insurance survey is generally performed for a very different purpose and with very different criteria than a purchase survey. Having made the mistake of using a surveyor who had previously done an insurance survey on the boat, I strongly recommend that you get your own surveyor.
Also, while you will not race the boat yourself, the quality of the sails on a boat like this is a major component in setting the resale price of the boat to the next guy. Since even good used sails for a Cat 27 are roughly a 1/4 to a 1/3 of the overall value of the boat, this becomes especially significant.