My dad is considering a used S80, I have four current Volvos, newest a 2000 V70R and oldest a 1970 142s. I don't know much about the diffences as far as dependability and maintainence between the different trim lines. We would most likely be looking at a 99 or 00 model, I would think the non-turbo 2.9 would be the best bet?

Steer clear from any '99, '00. ETMs don't improve until the '02 models.

Buying a used S80 is a subject that deserves a lot of research. The 99 S80 was apparently one of the most troubled cars ever made - any brand. It has components that not only fail but fail over and over again. They got better every year but these were very troubled cars. But do the research, including the archives here - I think there's a pretty good chance you're going to change your mind.

Stay away from the '99 and '00 S80.
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'88 240, '92 745, '98 v70 John, Tampa Bay

So avoid pre '02 models... what about which model? Is the 2.9 non turbo less problematic than a turbo model?

If it was me, I would stay away from any S80 unless I had an extended warranty thru volvo, not just any extended warranty, but a volvo VIP warranty.

The S80 is a beautiful car, and the turbo has tons of power, BUT that said the S80 is a highly complex vehicle with lots of electronic gizmos that depend on computer modules to work, and that's what makes this car unreliable and expensive. Cosumer Reports rates the S80 as a CAR TO AVOID. Diagnosis and repairs are very expensive, since it can only be serviced by the dealer.

The S60 is a more reliable car, not as luxurious or complex as the S80, but less troublesome.
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'88 240, '92 745, '98 v70 John, Tampa Bay

What about a S90 or a 960? Any problems or problem models on those?

2001 and up are ok to consider. I think the newer ones are pretty reliable and Consumer Reports changed its rating based on the improvements. I think they now recommend the car. There are a lot of happy S80 owners, and it's a great car, so I wouldn't throw it out just yet. Get a good one of a newer year (01 and up) that's been maintained and it should perform like any other Volvo.