99 Buick Century - Auto - 6 Cyl, 143,000 miles

I have a 99 Buick Century - Automatic -with approximately 143,000 miles on it. I have been going through a lot of coolant and have begun to notice a coolant smell when the car is sitting, but I have been unable to find any leaks. It has gotten hot a couple of times, but when I let it cool down and add coolant, it is fine for a while. Recently my service engine soon light came on so I decided to check the oil, water, etc. And I found that my oil looked like brown paint. My brother-in-law says that it has a blown head gasket and that both of them need replaced. He estimated that it would probably cost me about $1200. Clearly a lot of money. Do you agree that the head gasket is the problem and that that is what it should probably cost to repair?