Not all older C-30's have a rotted out keel stub and/or compression post block. If they didn't have a wet bilge all the time these problems were far less likely. I'd guess less likely on a freshwater boat that got pulled for winters too. A survey will tell you if it's a problem and if it is, just pick another C-30 - there's a lot of them out there. Or fix it yourself, if you're even halfway decent with glass.
The boat in the ad looks very good to me. It's got recovered cushions, which is a big upgrade from the horrid 1980 look. The diesel is a plus (1980 could have still had a A-4). Somebody has redone the electrical panel, that may be a good thing or a real bad thing depending on how well they did it. Outside looks quite nice for a 1980. I don't particularly like carpet on a boat, but lots of folks do on the curved floor of the C-30.
That curved traveler positively sucks if you like to actually trim the main whilst sailing. Sending Garhauer a reasonable amount of $$$ solves that problem.
Some other common early C-30 trouble spots:
The original chainplates, for the lowers were weak. Catalina Direct has a upgrade kit.
The rudder tube wears - pretty easy fix if the boat is out of the water. Easy to dx, does the rudder move side-to-side.
Wiring can be an issue - Catalina did not use tinned wire and water (particularly salt water) could do quite a bit of damage.
The "trailer plug" used in the engine wiring often goes bad.
Then the stuff that any boat could have - sails, engine, core rot, leaky portlights, etc....