This should help.
It is really only a 5 minute adjustment from inside the car .
http://my350z.com/forum/engine-drivetrain-and-forced-induction-diy/477784-shifter-re-alignment.html
Preowned engines in stock and good to go same/next day: £1500.00
Why take the chance mate, best to fit a quality known brand, fully synth, 5W30.
Alex.
In answer to your question it is most unusual for head gaskets to fail.
More than likely it is just as the guys have said or it may be an easily fixed breathing issue.
I would not be to unduly worried with your findings and providing there is no oil in the coolant or the engine oil is not emulsified then I would be reasonably comfortable.
Perhaps you could use your findings to negotiate on the asking price.
Alex.