Because of tariffs, you'll often find a 9.9hp motor rather than a 10hp motor. And that's plenty of motor for your boat--but you want a LONG SHAFT engine so the prop stays below the waterline as your boat hobbyhorses in rough water. Long shaft engines are less common. The prop should also be matched for a sailboat (slow than powerboats) so do a little asking when you shop, don't assume they are all the same.
Because the weight of the engine will be all the way at the end of the lever arm, you also will be better off with the lightest engine you can find. Don't fret over five pounds, but if there is a 20-40 pound difference, invest in the lighter engine if everything else (except the price) is the same.
Many boats have a slight crack between the hull and keel, just from production not being perfect and fairing not being perfect. If the keel bolts are solid, and there is no sign of damage, it probably is normal. Some brands like C&C are known the "the C&C smile" a perpetual crack at that point because they are built light for racing and the keels often wiggle just enough to keep that crack in the paint.
When in doubt...as others have said, get a survey. And consider asking the past owner to write on the bill of sale "no known groundings or known keel damage", or similar words. If he gets antsy about that...odds are he knows there was a problem.