Do a search on this topic and you'll find many discussion on reliability/performance issues associated with the different A6 options. A quick synopsis...
Performance
- Stock, the 2.7T offers the best acceleration when paired with the available 6 speed
- Modified (which you can do pretty heavily), it's even faster
- 4.2 has the edge in reliability and offers very smooth, linear delivery of power
- 4.2 likes to drink gas more agressively
- 3.0 is a solid performer if you're not a performance hound
- 2.8 is nice, but short on power compared to the others (that's why Audi moved to the 3.0)
Reliability
- 4.2 offers the best reliability
- 2.7T reliability is good, if you take good care of it. (here's a post discussing this issue:
click here)
- 3.0 and 2.8 are probably in between the two
The Sport option is definitely not a kit. Three things in it (which of the three you get, depends on the year):
- Sport suspension (20mm lower, 30% stiffer springs, 40% improved damping)
- Sport wheels (16" or 17" depending on year)
- Sport seats (drive in these once and you will want them)
Good luck with your search. You may want to follow the link in my signature to my registry page where you'll find some good background reading for people who are shopping.