2002buick lesbre

I bought this car girl said it was the freeze plug i think its
beyond that like head gasket cracked block it is leaking
pretty bad from bk of engine and overheating after driving
7to10 miles it has burning smell when u park and get out
she said that the lines froze last winter when parked so
thats why she thinks freeze plug. What would be the way i
can tell which one?and can i use that bluedevil stuff or
something steel it has lifetime warranty how can u tell if
motor is no good???