Mechanic tells me that engine in my 1986 model may be leaking on 1990 Toyota Cressida

My car has only 140,000 miles. If engine is leaking, is it wise to use stop leak additives? Do they work? What questions should I ask to ensure that it is in fact an Engine Leak and not gasket, etc.

Asked by for the 1990 Toyota Cressida
is this a coolant leak? oil? power steering fluid? or
transmission fluid? with any of these leaks, stop leak is not a repair. on a 25 year old car, how much money would you spend to fix a leak? at what point does the repair exceed the value of the car?