Blown head gasket or head case?

I just bought a car (1996 Chevy Monte Carlo) and I was doing an oil change and saw that the oil is a creamy color (like coffee with creamer in it). I know enough to know that's not normal, so I asked the guy at the part store what it was. He said it was probably antifreeze/coolant that leaked into the block, which would mean either a blown head gasket, cracked head case, or a blown intake manifold gasket. He said to test is using a compression tester and it should point me to the right problem (and therefore to the right solution). Is this what I would need to do? If it's the head case, how much should something like that cost? I am really hoping its not the head case because I've heard it's really expensive...If it's the head gasket, it should be pretty easy to fix, but what if it's the intake manifold gasket? I looked the price on those and it ranges from about $45 to $90. How much work would it be to do that? I am really disappointed in myself for not checking this before I bought the car (I always do, but for some reason I didn't this time...) but it's too late now and I've got to get this thing fixed...