I'm new to audi's and i'm looking to buy a 1998-2001 A4 1.8t Quattro. I want to be able to modify the car, things like chip, upgraded turbo, intake exhaust..etc. My question is what mileage should I stay under when buying so that it doesn't blow up when modified a little? 60k,70k, 80k? Opinions are all I'm looking for and would be really helpfull....

You car will not blow up. You may need to replace things sooner on a higher milage car but if you get a higher milage car you can always get new stronger rods for around $300 and new rings for $100 and take your car to get honed and then you will have a new engine and able to take serious power.

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FACTS > followers
197whp and 292wtq with no chip on stock ic, airbox and catback
AEG crank +Scat rods + 83mm Wiseco pistons = CincyTT Version 2.0T

Moding an Audi is not about if it blows up, but if you have enough cash to do it.