Re: Transmission noise
Why was the torque converter replaced at less than 39k?
That's hardly typical, and without knowing how it failed, you can't be sure that it didn't send shrapnel through the rest of the transmission.
The fluid would have been drained as part of the replacement, so you're looking at fluid with 600 miles on it. It won't tell you anything useful.
I'd probably negotiate a price that works even if the transmission needs to be rebuilt. You could also try talking it over with a specialist (there are folks that do little but work on the c6, e40d, 4r100. they may have some ideas).
'99 EB ex ENG KSWB news van, low rent 4x4 conversion (mostly fixed by now), home built interior.