If you go with a 98 vs 99 you will avoid having to deal with the dreaded drive by wire ETM (electronic throttle module) throttle body. They have a very high failure rate, the replacements are not any better, and are very expensive items. 98 was the last year for the traditional cable operated throttle body. 99 through 2001 cars have the ETM problem.
Watch out for AWD cars with different tire types or ones that have had tires changed in pairs vs. a full set of four. The bevel gear in the AWD setup is apparently very sensitive to tire diameter mismatches fornt to rear and can be quickly destroyed if this condition exists, repotededly in less than 1000 miles. Another big ticket repair item.
At 6 to 7 years old, and greater than about 60K miles, the timing belt should have been recently changed, or if you are buying from a dealer, should be a condition of sale. The 5 cylinder enginers are an interference type, and will end up with trashed valves if you lose a belt.
If you are in the vicinity of SE CT, I highly recommend you check out Eurocars Plus in Groton. They deal only in used Volvo sales and Volvo service, offer a 1 year/12K power train and 3 month/5K overall warranty. Very experienced Volvo techs in the service bay, and they thoroghly go over their cars before putting them on sale. They have a very good reputation around here.
I recently bought a 105K 98 V70XC from them at a very fair retail price. Virtually all the routine wear/age related items were replaced with new as sold - new rotors and pads all round, new Conti Contacts, new battery, timing belt, plugs, wires, rotor, cap, upper engine vibration mount, all filters (oil, air, fuel, cabin air), wiper blades, and even floor mats. Oil change and coolant flush. Pre sale detailing was also top notch, with the car looking like new.
Shameless plug qualification: I am not otherwise connected with Eurocars except as a satisfied customer that has been dealing with them for over 7 years!
Chris Bowne Stonington, CT