Unless you have proof that they have already been replaced, you should also change the Tensioner & the Tensioner Pulley in addition to the Water Pump.

In short, you need to replace all the moving parts except the cam & crank pulleys. If you want the new parts to last another 140,000 miles, you should also use only the OEM parts, excepting the Tensioner, where the Febi-Bilstein part is (I think) as good as the Aisin OEM.

The Volvo Water Pump for the late 960/S90/V90 did get pretty expensive ~$140. This pump was made by Aisin. You can still get the Aisin pump from Eeuroparts.com at ~$81. I have used this pump and it is virtually identical to the original. You can even see where they ground off the "Volvo" from the casting. The only difference I saw was that the stainless steel impeller looked electro-polished on the Volvo part and was not electro-polished on the Aisin 'aftermarket' part.

Tasca Volvo (getvolvoparts.com) is a good source of the OEM parts.

The whole deal is about $400 in parts with shipping.