I just bought the little brother, the SJ21. Built by Clark from early 70s to mid 80s. The boats share many traits: quick rather than fast, though still competitive in their class, a blast on a reach or run (planing hulls) but not the greatest pointers, decent build quality, good for protected waters or inland lakes. Trailerable, easy setup, very low-cost to maintain. Tender: the small SJs have a reputation for being tippy, but forgiving. Ve vill learn rounding-oop!
Rigging is simple on these boats, but they have been much-modified by their owners and can often benefit from updates and rationalizing. Biggest problems are osmosis blisters if the boat was in salt water; and rot in the balsa cores. Check for sponginess around the mast step, front hatch, pulpit, and chainstays. The hardware was not well-bedded and SJs often exhibit wet balsa around the fasteners. Fixing it can be a minor -- or a major -- project.
The SJ23 has active owners' groups and a Yahoo page. Tends not to make fleet numbers at one-design races, but lots of people sail them in PHRF or Beer Can regattas. You can read owner reviews on SailNet here.
Hope you got a good 'un, and keep us updated!