Before you hire a surveyor which is expensive, I recommend purchasing the book "Buying & Inspecting the Aging Sailboat" by Don Casey, which is an excellent guide to follow when first inspecting the potential prospect C 30. Read our C 30 forum & older posts will show you reoccurring themes: compression post rot at the base of the mast post. Corroding chain plate & keel bolt connections, rot occurring in the plywood sandwiched between the keel & keel bolts, & associated Catalina smile cracks at the hull to keel joint that are directly associated with the plywood bilge plate rot. These are problems specific to the C 30 design flaws, but are not unique to Catalina boats.
The C-30 is a very durable & forgiving design & has stood the test of time. But like any production coastal cruiser, she has her limitations. Generally the older the vintage, the more problems you will have associated with natural deterioration from age & lack of regular maintenance. That said, I would buy an older C 30 that was well maintained, over a newer C 30 that has been neglected. There are so many things to consider:
post 1982 boats will generally have more reliable Universal diesel engines, stainless steel keel bolts & upgraded chain plate connections. Only the newest C 30's do not have the wooden compression post block at the base of the mast that is prone to wet rot from bilge water exposure. Newer boats will have improved wiring harness & electrical panels. Shop around, there are a lot of em out there (over 7,000!) since they are the most popular 30 footer ever built. The pluses are they are very beamy, roomy, forgiving under sail & have a stout solid glass hull & heavy cored deck structure.
Watch out for wet rot deck core & delamination (found by sounding the deck with a rubber mallet), worn out sails that costs thousands to replace, seized turnbuckles in the standing rigging, original wooden spreaders that rot over time, excessive play in the rudder, & avoid any original gate valves at the through hulls, etc.
Good luck & enjoy the hunt, my favorite past time. Cheers.