Off the top my head I would think:
- IMMO may need upgrading
- CCM may need upgrading
- antennas and cables [there were 7 or 8 in my 2005 A8]
- adaptation via VCDS
Newer models can handle 5 keyfobs, not sure about a 2000.
May want to see if anyone has done this retrofit in the model specific forum.
Jack over at www.FixMyEuro.com
would certainly know the answer to this as he's one of the few Audi approved locksmiths.