rust on a cast solid steel
keel shouldn't present a problem....clean, seal and paint
but the joint is another matter. it appears to my eye, I see some rust spots at the joint. it's also unusual to see a caulking bead along the exterior of this joint, suggesting that this bead was put there after the original keel connection to hide something.
, when it rusts, also expands, and as an example, this expansion can cause a steel re-bar to pop concrete of a concrete beam.
in the case of the hull/keel joint, this expansion needs be only wafer thin to cause a separation to allow salt water
ingress into this joint and cause a problem. Caulking can hide the problem, but not fix the problem. if this situation has been there for some time, the problem will grow proportionally.
additionally, a hard grounding can also be a problem for a keel/hull joint.
this boat, underbody sight unseen, is not a smart move in my opinion.
getting a boat surveyed, generally also involves hauling the boat out. yes, this is pain in the butt and an additional expense that you cannot recoup, should you decide to walk from the deal. boatyards
that do this, typically hold the boat in the slings and give it a quick wash so as to better see things.
you don't have to be present, but certainly have an independent (not a seller's) surveyor take a look at this boat.
as LittleWing, above, suggests, do yourself a VERY big favor and get this boat surveyed, no ifs, ands and buts about it !!!!!