What is steel and what is alu on A8?
 

Considering buying one of the first versions of the A8 from the 90's or 00's. How much of the car is made of aluminium? And what can I expect to rust?

the things that will rust, especially in the northeast road salt are:
transmission lines
power steering lines
K frames
fuel and brake lines

the rest of the car is aluminum. aluminum can corrode, but from what I've seen it's not significant.
you can see the power steering lines behind the driver front wheel
the transmission lines will rust near the radiator
the K frames connect the driver side and passenger side lower suspension together. I've not seen a rusted out K frame but it could happen.
watch for broken springs too.

correction - brake and fuel lines are aluminum
 

the brake and fuel lines I took off my 1999 A8 parts car are aluminum