the auction inspector notes state "engine noise".

should go super-cheap and leave room for major upgrades but I am wondering, with 50k miles, what noise could this be?

I know, total speculation, but thought there might some common ailments I should look for. The inspectors are pretty good though at mentioning turbo noise versus general engine noise. So I am thinking lifters or bearing as worse case scenario. Water pump or pulley at best case.

any thoughts or insight appreciated!

I would do; get an Audi Tech (if you know one, or pay a Tech for this service) and have him checked out the car, assuming you won't be able to move it from its current place.
Ask the tech to bring a stethoscope, in case the source of the noise is not so obvious to determine.
If the car is really that cheap then it might be worth a small investment.

Otherwise your guess is as good as anyone else.