Rust is the main issue. Ask about the maintenance record. You need to know what oil has been in this car before. Ask when the timing belt was last changed and if the rollers were changed too as you might need to budget for that. Disconnect the lower intercooler hose and see how much oil there is in there, a couple teaspoons is alright, a quart might indicate a failing turbo seal.
Look what kind of coolant is in there, the good stuff is pinkish. Usually if it was serviced by someone not knowing what they are doing you'll find your usual green coolant. This might be an indicator other maintenance might not have be properly conducted.
Other than that the engine should pull alright to 5000 rpm, but don't worry too much if it doesn't, as long as the engine sounds good and smooth. Lots of small easily fixable issues can cause that so you can negociate the price down. Pull out the rubber hose going in the egr valve bolted to the intake manifold. Look how plugged the intake is, it might just need to be cleaned to restore performance >3000 rpm.
All the good stuff is in there
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Oh btw are we talking about an early 99 (97-99 model) or a 99.5 (99.5-2003)?