Oil in expansion tank 300E

I have a 93 300E2.8, 64,000 miles, owned since new, always maintained properly, runs perfectly. This afternoon I noticed some gunk in the radiator expansion tank and it appears to be oil in the water. Nothing in the oil; I checked and rechecked that. I guess I assume it is a head gasket but is there anything else I should check before spending the big bucks for a head gasket?

Any estimates on what a shop would charge to replace the head gasket? (I am am normally a "do it yourselfer" but not sure I feel up to this.)