Hey matey - lots of D2 owners here who might be more tempted in your offer too
The main issue I have with the gearbox isn't the likelihood of it failing - you can potentially hit the limiter in 5th gear and it's revving like crazy as it is. To make it usable it'd be handy if the box was a 6 speed to make it more refined on a run. D2 boxes are its known weakness, whereas the D3 on seem more robust and run more sensible gearing.
As with any mod that adds power, if you use it to its full potential all of the time you'll put more load on other components.
The D2 S8 isn't quite RS6-bad as far as its ability to cope with more power, or even the power it has as standard! Doesn't help that Audi have never specified a service schedule for its auto boxes.....
Adrian