Catalina 30 - minor cracks in keel??
Sight unseen, the questions you are asking are really the ones you need to ask the surveyor.
The small cracks in the keel, which may or may not be so small if a diver can see them in the generally turbid and dark water of a marina ....may be more serious than meets the eye. You would definitely want to have the keel and its mounts inspected closely. If every thing is in pretty good shape, then your repair costs shouldn''t be all that much especially if you do the work yourself. Otherwise it will be based on quotation. On the other hand...they may just be stress cracks that need repair.
I might suggest that even if the surveyor says the keel situation is ok....get with the seller of the boat and tell them you want to escrow at least $1500.00 pending an assessment of the keel out of the water by mechanics who would do any repairs on it.
That way, "the sweetness of the low price will not turn into the bitter taste of hidden repair costs after the sale is complete", and you have no available recourse after the fact.