Quattro Coupe 1990

[Tony Street <tstreet@intacc.net>]

I currently own a 4000S and am considering purchase of a 1990 Quattro. 
Would like to know:

a) anything particular to look out for on this year and model which has 
proved problematic?
b) how expensive are they to maintain (ignoring oil changes, brake pads 
and similar consumables)? What are the biggies and how often do they go 
(eg. Air/C, clutch, timing chain or belt, etc.) ?
c) how reliable are they? Do things wear out gradually and provide some 
warning or do they tend to go suddenly?
d) how long will a properly maintained engine and drivetrain last ? (eg 
using synthetic oil changed every 3000 miles) 

Thanks in advance for any advice you can offer.
Tony