Water and coolant mixing together

[Darrell Jennings]

I have a 2001 C2 cabriolet with about 100,000 miles that has a problem.  It's been a bullet proof car since new, but last month I spent a lot to replace an oil separator and to flush out the cooling system.  Now a month later I still have oil in the coolant and Thompson service is telling me that it is either: (A) a bad oil cooler, (B) leaking "frost" plugs, © a bad head gasket which they say means an engine rebuild, or (D) a cracked cylinder or block meaning a new engine.

To check the oil cooler they want $900, plus another $500 if that's what's wrong, but I'm concerned about throwing more money at it if it in fact is a major engine problem that costs more to fix than the car is worth. 

Any suggestions on great mechanics that might give a second opinion or other options Thompson's not thinking of?