If the faces of the surfaces are in good condition and true, then no sealant is required. The
gasket will be sufficient.
Don't worry too much about an exhaust leak with a
diesel engine. First there is only a small amount of CO in a
diesel exhaust when compared to a petrol (gas)
engine. Secondly the smell of the smallest leak of in a diesel exhaust will alert you a long time before it becomes dangerous.
Put another way, the smell of a small leak in a diesel exhaust leak drive you out the
cabin quick smart!