1998 Buick Century Recently Acquired Limited 3 1 Ltr V6 Sister Husband They

I recently acquired a 1998 Buick Century Limited 3.1 Ltr V6 from my sister and her husband as they were getting a new car. When I checked the oil it was 1Qt over full and clean, the trans. Fluid was ok and also clean, the coolant reservoir was empty, after driving it for a few days I checked the coolant reservoir again, it was brown and slimy as if contaminated with oil, does this mean I have a blown head gasket, or could there be some other reason for the brown slime. My brother in-law said he once put the wrong coolant in the radiator and reservoir but his mechanic cleaned it out.