How does one know the condition of a used engine? Milage and compression? The milage is undeterminable (e.g. all JDM engine have 30,000 Km) and cold compression is not accurate. How about plain bearings condition and oil pressure? How about valve stem seals and seal surfaces? How about the cylinder head gasket and the integrity of other mating surfaces? None of these can be determined w/o staring and warming up the motor and doing some disassembly (or vice-versa)
Putting an old motor is car w/o going through it is a gamble. Winning is far less common in gambling than loosing. And then, of course, in the case of turbocharged engnine is the condition of the compressor(s), i.e., bearings and impellers.