Jay Jenkins at The Yankee Trading Company here (Volvo restoration business) just told me the '89 700 series turbo models are hard to beat.

my reading leads me to believe the turbo has a lifetime of about 150,000 miles. The Garrett turbo is more suited to the highway (spools up later) and the Mitsubishi better for in-town driving. Doubt either are well served in stop & go driving. Oil and water cooled models are better than just water cooled. An '89 probably has the Garrett T3 with both water/oil cooling.

Check for presence of oil in the intercooler. there is a draincock under it for letting out any accumulated oil. some is O.K., a lot is not. Check into the hoses. Look underneath for abrasion and wear through. These are expensive. Look for oil, also.

While there is no flame trap, there is a plastic "Y" connector between #3 & #4 intake. Check it for plugging. Crankcase ventilation is important to turbo life.

Make sure the front seats glide well. The mechanisms are more complicated (contain rods and ball bearings). If ever you need to take one apart, its rediculously involved.

The car won't take the 14" wheels from a 200 series, if it came equipped with 15" wheels (larger brakes)

Use synthetic oil. Sludge (and avoiding it) is key to turbo longevity. Oil supply& return lines tend to plug. Replace these when the turbo is removed for something.

Look for and inquire about any coolant leaking into the passenger side. It means a bad heater core. No fix. See if the temp control valve near the brake booster is black plastic or mostly metal. Metal won't give a catastrophic failure with loss of coolant, like the plastic OEM does.