[Fibbado]

Most people just keep the thought in the back of their heads "It could break at any time" and they plan accordingly. This is of course much less likely with an A4 than a T56 which is a very tough trans. Many people run 12's with their stock 10 bolts. If you plan on upping the cars power (which is very easily done) or putting better tires on it with some gears or what not then think about the rearend.

Aside from the rearend as far as maintenance goes it's pretty normal stuff, F-bodies are about as complicated as a hammer so if you are mechanically competent you can do most anything yourself. I am at 130,000 miles on my LS1 and when I took the heads off the cylinders still had crosshatching. Always run premium I reccomend. Keep an eye on your oil level, check it between oil changes, if you notice oil consumption thats normal you could just change to German Castrol that might help. With only 50,000 miles on the car you are looking at I would say you don't have much to worry about assuming the previous owner didn't abuse the car too much, a little abuse is okay (It's and LS1 it was born to scream )

As for year differences the newer the better, 2001+ had the better Master Cylinder and Slave Cylinder as well as LS6 intake manifold. Little known would be the fact that 2001+ had better internal components to the T56 six speed.

These cars are tough, they are fun and they are fast. Respect them and think about stuff before it goes wrong.