Look up the International Catalina 30 Association and check out the ton of information on the 30 model from improvements by model and hull number over the years to downloading an Owners Manual. Its all there!
To date over 7,000 hulls have been produced and improvements in power, electrical packages, interior arrangement, keel changes, standing and running rigging, etc, are all in the 30's evolution. I own a 1988 Mark II and through my reserach found that the Mark II model came with big improvements that I was seeking. I am very pleased with this model. It really gets down to what you want to spend and how you plan on using the vessel. With Great Lakes sailing you may find a Mark II model with the two cylinder M-18 diesel that replaced the M-11 and came standard for a short time before the M-25 was introduced. Less power in tidal and current situations, but sufficient for your needs on Lake Superior. Tall rig versus Standard rig? Do you plan on racing or cruising? Wing keel for gungholing or deep keel for more open water cruising?
I can write pages on this vessel, but briefly, YOU MUST get a survey done after you find the model you like. Two big 30 issues are: compression post deterioration and rusting keel bolts and the "catalina smile". It was not until late in the 1988 Mark II model line that the compression post support unit was replaced with a metal stand. Stainless keel bolts might have been used prior to the Mark II model. For me the Mark II model is a better choice, but then folks love the older models that make up the workhorses of the Catalina fleet. Which ever 30 model you choose, they are sound investments, excellent sailors and as roomy as a 30-footer can get. You cannot go wrong with a Catalina 30. Do you homework and best of luck.
Bob
Lake Lanier, GA