Don't bother with 2001-2002. The "R" is fun and practical, but can be expensive to maintain properly (Nivomats, AWD angle gear).

1999 introduced electronic throttle and it was a bust until Volvo replaced the units with a 'yellow' label version. ETMs failed around 100K miles, so few are still on the road.
1999 also saw the start of coil over spark plugs. Each new coil is about $50.

2000 was the beginning of computerized instrument clusters and central buss modules, a long learning curve for Volvo.

Most of the V/XC70s come with at least low pressure turbos, quite reliable and much better than the 700/900 cars.

Rust is a thing of the past. Except for the trim under the rear window of the wagons. The trim piece gets rusty and the rust runs down on the paint and eats the paint way. Replacing the trim piece is not easy and must be done before the lift gate gets discolored.

Front wheel drive does everything with the front wheels, so check control arms, bearings, bushings, etc. Most struts last about 130K miles and people just don't replace them. Strut mounts bushings fail and need to be replaced with XC90 or heavy duty versions. Sway bar end links are weak and last a couple of years.

The AC evaporator is very difficult to replace. Prior to 1999, they tended to crack and leak from moisture build up. Make sure the AC works before buying.

There are more 'things' to look for, but these are very good cars and can last for several 100K miles IF maintained. More comfortable than the 200/700/900 in my opinion, with better power and reasonable mpg.
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My name is Klaus and I am a V♂lv♂holic