I also prefer the "doghouse" coach roof of the Mk1, and the larger cockpit for social sailing. But the interior is quite small compared to the Mk2, and waaaay smaller than the C22. SJ21s are mediocre build quality (like the C22), but their issues are simple to deal with. Even wet decks are more tedious than difficult to repair and can be ignored for years. Most SJ21s have or once had wet decks, and they are/were sailed anyhow.
Other known issues: Transom core around bottom gudgeon and motor mount may be wet. Fixed portlights leak. Mk2&3 have bulkheads that bear inspection for leaky chainplates. That's about it -- the reason these boats are still around is that they are cheap to own and a blast to sail. Here's a website
about a guy who painted his SJ21 (dark blue, no less!). Doesn't look too hard. We're going to try it one of these days.
Prices are as Joekano suggests; they've actually risen the past two years, for some reason. Figure average base price around $2k; less if it's been neglected and has wet decks, more if it has racing upgrades, new sails, and an outboard. $3000 is probably all any SJ21 is ever worth.
Performance is dinghy-like, very quick. Flat bottom and bluff entry means the boat will exceed hull speed if driven; also means it can pound a bit in chop and gets kinda squirrely on big following swells. Points well inside a narrow window (8-12 kts); its weatherly qualities diminish markedly outside that window. A genoa is useful in light wind areas, but the boat has a bit of lee helm and the 100% is a better choice above 12kts. We've sailed comfortably in 40 kts on reefed main only (boat loves main-only in a blow) and survived a 50+ knot squall, tho we don't advise it.
One person can step the mast. Unless you have strong currents, a 2hp motor will push it fine. We get 3kts from a trolling motor.
Keel retracts fully, so launch & retrieval are easy; we often tow with a 2.4l 4-cylinder van.
Overall, I'm really pleased with the sailing qualities of the SJ21, even if I occasionally want to choke the Clark Brothers. C22 has bigger fleets & better parts support (also a real nice bunch of people). Both are excellent boats for messing about in.