The 04 model isn't the best D3
A8 to buy because it has the most problems, 90k miles is a lot on an 04 for the reason audi didn't use synthetic oil in the audi V8 engines until 05 which means the 04 probably has a little more wear and if you start using synthetic it would leak like a grandma bleeding.
On the other hand i have seen a few 04
A8's with over 150k and they were still running strong so i guess it depends on how well it was taken care of. If you want the
A8 get a good record of its oil changes and major services.