Been a lot of good posts already, hope you have a printer running.
Open every locker - make the owner tell you specifically what stays with the boat and what leaves with him. List items you want to be sure stay. Understand that you will be cleaning his garbage off, once you buy it it is yours.
Ask the owner to show you how to operate every single system on the boat.
Do it twice. It's not just about sailing, it is also about understanding your boat and how every single piece of it merges into a entity underway. Other than finding another Catalina 30 owner you won't find someone to show you anything.
Catalina 30's are good boats, the Chevy Impala of the US sailing world. Check the ports for leaks, and see if the owner bothered to add a fuel filter (my buddies 1986 never had one).
Bother the hell out of the surveyor with questions, you are paying him so his time is your time.
Note the condition of things like the batteries, oil, etc.. if these vital parts are neglected imagine what the rest of it looks like.