These engines can leak and then they can LEAK.

There are many of them running around that leak a slight amount of oil down the side of the cylinder block due to improper head gasket sealing. There are probably some of these doing so and the owner doesn't even know. This leak you can live with for awhile by simpling keeping an eye on it.

A COOLANT leak is COMPLETELY different matter and must be dealt with immediately.

Yes the 12 to 18 covers the valve job. Could be a little more if there is a corrosion problem. If that is found don't buy another head, there are plenty of good shops around that will weld up the head and make it good as new. Mine was welded up well over 100,000 miles ago and is doing fine.

Good luck,