The "crack" is really a joint between the fiberglass hull and the bolt-on keel. It's not uncommon to see a crack in the fairing and barrier paint at the joint, but, I think, it isn't so typical to see it all the way around as this one shows. Obviously, somebody appeared to see a need to treat the joint at the front end and the back end of the keel. It's obvious they are not attempting to conceal it. The stains in the first picture could be troubling if the stains are originating from inside the separation - can't tell from these photos. If the bolts can be torqued to specification, that would be one indication that there are no worries. Another good sign would be if the bilge takes on no water while the boat is floating. That would take several days in the water with a dry bilge for me to be convinced there is no seepage. As with just about any old boat, there are some unknowns that you simply can't resolve during the quick decision interval that you have. If you are uncomfortable with the need to perform any repair, you are probably better off dismissing a boat with this type of uncertainty. If you are comfortable handling unknowns as they may show up, this may not be any concern at all.
This doesn't look like a catastrophic failure ready to happen. But it does look like something to watch with an eye toward the possible need for remedy.
