Further to Spook's thought on the timing belt. A relatively low mileage car in top shape such as yours may well have had a conservative owner and been dealer serviced at the suggested service intervals. Check local Volvo dealers with the VIN to see if they have records. It could save you a fair chunk of change in belts, idlers, tensioners and labor.

The TB service interval for your B6304 is 70K mi, so at 160K may well have been recently replaced, or it could be quite overdue. Unless you can verify both belt mileage and age then best to replace it. The two idler pulleys are sometimes routinely replaced every 2nd belt, the tensioner idler often with every belt, so both idlers may be due with the next belt or at least carefully checked to make sure they don't spin too easily. Of special interest is your automatic TB tensioner being checked and/or replaced, in addition to its idler pulley. To do that properly you probably need to use a belt gauge to make sure it's still maintaining tension to spec, else it should be replaced.

The serpentine drive belt does not have a service interval, just inspection. It also has an auto tensioner that should be checked at this age. You can usually tell an old belt by its looks, but with a low mileage belt it may look fresh. When folded both ways, there should be no hint of cracking along it's length.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now