How does the installation of a used engine effect resale value?

I was about to sell my car (2001 Audi A4 1.8T) when I started having engine trouble and my mechanic told me I had to replace the engine. Sigh.

Obviously, I want to save money and go with a used engine, but I realize it won’t be as dependable as a new or rebuilt engine.

Questions:

1. Can a buyer (or his mechanic) detect a replaced engine?

2. Are there any guidelines on how such an engine replacement would effect resale value? In other words, if the car is worth $10,000 with the original engine, how much less with the a newly installed used engine?

Thank-you!