Best Answer - Chosen by Voters
Alright. I owned this car, First thing to do, find one at a lower price. I sold my 1999 Audi A4 Quattro with the 1.8T with neuspeed exhaust, new turbo, ECU flashed, and intake for 5k.
Second, 140K miles is a lot of miles on these cars. The 1.8T isn't know for it's great reliability. Also, Audi isn't know for being cheap and easy to work on. I did a lot of my own mechanic work and trust me, when you get in there and everything is Allen bolts you'll curse whatever engineer design it that way.
Now, that being said, If you do CHOOSE to buy it there are a couple of things to consider.
First, check and see if they have all the maintance records. If they do find out when the timing belt was done. (The timing belt keeps the camshafts, rods that spin and move the valves to let air in, in time with the crank shaft, the big beefy hunk of metal that connects to the connecting rods). The 1.8T is notorious for having a the timing belt tensioner freeze up, which snaps the belt send the valves into the pistons. Blowing the whole motor.
A mechanic can do the timing belt, but it will cost about $1200. The water pump should be replaced at the same time just because it has to be removed from the car to get to the belt anyway. It's a $120 dollar part. I did this my self and had to remove the whole front end of the car to get to it. Bumper, radiator, radiator support beam, ac condensor.
Second, Get a carfax on it. Just because the deal says the car is in great condition doesn't mean it is. Last time I checked there where plenty of lawyer and used car jokes involved some layer of hell.
Third, Click here for the NADA price of the car. I assumed that you where buying an automatic and sunroof. http://www.nadaguides.com/usedcars.aspx?LI=1-21-1-5013-0-0-0&l=1&w=21&p=1&f=5014&m=1190&d=198&y=1998&c=16&vi=1525&z=34134&da=-1&mi=140000
MSN List of common problems
http://autos.msn.com/research/vip/Reliability.aspx?year=1998&make=Audi&model=A4&trimid=-1
In the future make sure to check carfax, MSN Auto's for list of possible problems, and NADA for car prices. Happy hunting, and if I were you I would find a nice Subaru legacy. Super reliable cars.