Back in its day, the 70's, the SJ-21 was an inexpensively-built boat. Hull to deck joint was mediocre.
OTOH, real Good sailor and fast, being a big dinghy-shape with a sit-down interior.
Better built & rigged than a Venture or a Clipper Marine, to name a couple from lower down that list.
The thing is, you can buy scads of boats from that era for under $2000., that are more easily certified as ready to sail and enjoy.
You've already read some great advice here, and I would only echo the reply about getting a survey on that boat (and any other you buy, even for near-free), for the education alone.