What is the best engine block sealant for my head gasket leak?

I have recently purchased a car and it has alot of problems. I suspect a head gasket leak or cylinder head crack and i cannot afford these expensive repairs. What kind of engine block sealant would be suitable based on the following symptoms, and what kind of risks should i consider before deciding to use the stop-leak? Symptoms: air that smells like gas being introduced into the coolant. and: (maybe) trace amounts of coolant in the engine oil. Thanks for your help