I just bought a 1984 Ericson 30+ in April, also in very good condition with many systems/parts replaced and upgraded. We sail in the Gulf Islands, Strait of Georgia in BC but live in Edmonton, Alberta so haven't had all that much time to become familiar with her. Other than the usual maintenance items (eg. replacing gate valves with proper seacocks, ensuring double stainless #300 clamps on all hoses, replacing steering cables, standing rigging, etc.), I heard from one 30+ owner that his cockpit hoses had developed a slight leak, which was hard to track down as they are fairly inaccessible--obviously this could cause eventual sinking, so not to be ignored.
On my boat, the compression post had been redone professionally with a steel plate added on the underside of the deck inside the headliner, and additional support added below the sole; this seems to be a fairly common problem on various size Ericsons.
I recently read Nigel Calder's text entitled something like "Electrical and Mechanical manual" for sailboats, about 800 pages. It goes through every system, describing how it works, identifying potential problems, listing regular maintenance requirements, and troubleshooting repairs. While it is "dry" reading, it is a wealth of information, which has resulted in a mile long list of "to do's" to check and adjust on my boat. I would highly recommend it.
It will be interesting to see how other 30+ owners respond, as this topic is quite timely and of interest to me, given our recent purchase.
Best wishes!
Frank.