There are lots of places to get rigging information, mostly from the various sailmaker's web-sites in the form of a tuning guide. Harken used to produce a great diagram to help you figure out how to buy more boat stuff, but I think the best place now is aps- APS - J/24 Boat Photos for Reference
This has a collection of photos from two different boats showing examples of ways to do different things, anything from motor mounts to the genoa shackle. You'll want to go through the tuning guides (my favorite is the one from Quantum, here- Class Details - Quantum Sail Design Group :: Premier Sail Design and Development
) and watch the various online videos taken over the years by Harken and a few others (all on youtube). I'm sure you may have stumbled on the class website- http://www.j24class.org/
, there is a great first-time buyers page there with bunches of hints of what to look at. I'm pretty sure core rot down into the hull, while possible, is pretty rare- definitely worth taking a look at, though. By far the most likely scenario for core rot there would be if a boat got holed, and a repair didn't adequately seal off the area and water got in. Core rot is very common in most decks, but depending on where it is (and what other boats you have in a similar price range) may not be a deal breaker. I know the j24 class web site says sails don't matter, but sails aren't cheap. Obviously, the boat is a lot harder to fix, but if you are looking to race, a newer set of sails can really sweaten the deal, especially a new genoa. Capsizing and turning turtle is really tough in a J24. I've never done it, but we've broached quite a bit- always with spinnaker up (my own boat and crewing on several others). The one thing to be sure of that everybody will tell you, is never go out (in any wind) without securing the cockpit lockers with a clip that cannot accidentally open. No big deal, but you could lose your boat. It's always a good idea to keep duct tape on the boat for a few other reasons (mast boot, for example), but sealing up the hatches means that you are probably done sailing, right? How often is that going to happen??