Common problem areas are the ABS module and ETM:

ABS module repair(I think $100-$150, and once Vic fixes it you should be OK):
http://home.earthlink.net/~vicrocha/Volvo_Parts/Volvo_V70_ABS.htm
ETM module weakness:
http://vexedvolvo.org/

Note that the ETM module was not a good design. I think the mechanical failure rate by 100,000 miles was about 100%. So, they found a way with software to mask the hardware wear problem. People here have said the software fix eliminated any problems they may have had. The hardware problem was never corrected (if they replace the hardware module, you get the same design) as far as I read on this site. To me, I would avoid a purchase of this vehicle only due to the discussion on the vexedvolvo site. I appreciate that people here are driving these cars without problems, but after reading how the potentiometers wear, I would not want to deal with the long term consequences. I think they now warranty them to 200,000K, but hey, what if the software fix takes you to 201,000mi, and then the darned hardware becomes so bad the software can no longer compensate? Just my opinion after visiting that site. You may want to read the materials on there carefully (takes about 1-2 hours) and decide how you feel about it.

My wife's 1999 V70 has been remarkably trouble free. I'm really pleased and a bit surprised. Beware of the delaminating door panels, a cosmetic weakness. No frills is a wise choice for used cars. I hope no frills means manual transmission....

With the 99 model year you need to be concerned about the ETM.
Here is a thread that covers the topic

http://forums.swedespeed.com/zerothread?id=39438

Volvo now covers the ETM up to 200,000 miles and offers a software upgrade that reduces the dithering affect wear. Still be aware and figure it into the price. My 99 has been a wonderful car. The dealer addressed the ETM issue as part of the purchase.

You will also encounter ABS module issues (easy to fix) and all of the usual front end "challenges".