I'm looking to buy an 1998 Audi A4 1.8T Quattro with 84k on it,is it a good car?

What im hearing from your description, dont buy it. First of all, 84k miles! an average car only lasts around 95-100, and 5000 for a car with 84k miles, no way. I dont even know about this specific car and im turned off to it. My advice is no, especially for someone who likes to use it alot.

that is a decent price as for cost of maintenance it is high. if you are just look9ng for bask transportation there are cheaper options out there but if you are looking for a german sports sedan (Audi BMW or MB) the A4 is one of the more reliable and cheaper to own German sports sedans. The mail thing you need to look at in this car is for any transmission seal leaks (cheap fix but a good way to negotiate the price down) the not so cheap fixes that you have to ask about are if the timing belt and water pump have been done this can cost about $1000 and it is due if it hasn't been done (the dealer may tell you it doesn't have to be done till 120K miles but most (if not all) Audi enthusiast will tell you to have it done between 70K and 80K miles b/c if they break the repair bill will look very similar to the purchase price of the car. the other thing to look at is the turbos they have been known to fail between 70-120K miles and that is anouther expensive repair all in all they are a great car and other than the cost of the timing belt and turbo replacement not too bad to own. if the turbo is original once you buy the car set aside $20to $50 a month so that when it goes bad you can replace it without worrying about how to pay for it. the other option is if it is original turbo look for a A4 with the 2.8 V6 its a lower maintenance engine.

Make sure that you are figuring on the added cost of premium fuel and synthetic oil changes (the oil changes run about $90 to have it done or $60 if you do it yourself which is more expensive than a $20 oil change at the jiffy lube)

All in all they are a great car but be sure you can afford to maintain it before you buy it people buy used German cars all the time then complain about how much they cost to maintain they were designed for people with money who are not phased by $100 oil changes etc. they just want the performance and the cost is secondary.

what are some things that usually go bad on this car?

timing belts, MAF sensor, vac leaks, a long list of electronic problems. all are expensive fixes (if you intend to fix them at the dealer). I just got my MAF and vac leak fixed at dealer.. cost $1,300. The parts cost about $400. The labor charges are crazy.

for 84,000 miles. I would say the car is worth $5000-$6000. It's a good deal for a audi. But be ready to spend $1000 at any given time. They are solid cars, but you need to do preventative maintenence.

As for the warrenty. Who is issuing it? Is it the AUDI dealer or is it a used car dealer? If it is the audi dealer then your fine.

first off cars typically last much longer then 100k and audi's are expensive to begin with so $5000 is not bad but thats beside the point......Audi's are extremely expensive to repair and parts are also extremely expensive. A friend of mines dad owned one for a short while and when his head light went out it cost him over $300 for it to be replaced. For just one headlight.