is it worth it to get an used engine?

I have an audi a4 1.8T  i took it to the mechanic to replace my thermostat they
already replace it. it turns out that my engine oil light was on and i had oil
sludge  he told me that it was bad that it allowing to drain and allowing new oil
circulate it got so  bad that he said a needed a new engine i was wondering if i
should replace the whole engine or leave at is.