I own a 1984 Catalina 30, and it's a great boat for what I got it for, a starter boat to bring the Admiral along and a coastal cruiser. For the length, it has a lot of room below, largely due to the wide beam. I love my Catalina, but like any tool you need to understand the limitations. I'll be parting with it and upgrading when/if we decide to start going further offshore.
There are some good reasons above for why people don't consider it appropriate for serious offshore work, and they are true. However, what no-one has mentioned yet is, for me, the real show-stopper. The hull-deck joint is *only* secured by adhesive. There are no through-fasteners of any kind. That's why she gets the nickname "the tupperware boat." The lid is just sort of stuck on. Leaking through the hull-deck joint is a very common problem in the C30, and if you decide to buy one, I'd recommend you spray around the joint with a hose and then check inside for leaking.
Before we got this boat, I found a killer deal in Marina Del Rey for a nice 79 C30 tall for 10k. So you can find a deal. Generally you'll probably end up parting with $18k - $20k for something you'd like to have (or you'll put whatever the difference is between $20k and the purchase price into it to make into something you'd like to keep.)
Good luck, and if you have any more questions I'd join up with the yahoo Catalina30 group. Ton's of good info there.
JRM
"Imagine"