I have '96 A4 quattro 2.8 12v engine, A '98 has probably the 30v engine. '98 was the year the 30v came out in the US, I believe. Between the two, the 12v is whole lot simpler and more reliable with less to break (12 valves versus 30 valves, DOHC with chains and tensioners!), but less powerful.
I had no issues with the suspension, others have had a nightmare with control arms all around, and I think the A4 has 16 of them. Wheel bearings are a problems for many A4 owners. I think one of mine may be going now.
The MT has a fragile first gear. Be gentle with the first gear. If the tranny is noisy in first gear, stay away from that car. The dealer won't be able to fix it properly (went through it twice!).
Your A4 will require timing belt (water pump, thermostat, tensioner, serp and timing belt) service every 4 years or about every 70k miles. That figure can be debated, but with an interference motor, it's best no to gamble.
At the mileage at that you are looking, you will probably need new valve stem guides/seals eventually. Find out if oil consumption on the vehicle is high. Some niggling problems will occur, but with preventive maintenance they are generally not much of an issue. Don't get me wrong, and older A4 is a high-maintenance vehicle, but it does respond very much to how it's being treated. A well-maintained A4 will be a pleasure, a neglected one will be an expensive nightmare.
In short, I recommend a medium mileage (125-150k mi) A4 only if you are a hands-on guy and don't mind doing a fair amount of preventive maintenance, or if you don't mind spending a lot on labor.
In nine years, and over 144k miles, my A4 has not once broken down or become undriveable.
