I'm with the 'every other cambelt change' group re the water pump replacement - and its Volvo only branded for me too. I have had two water pumps weeping from the tell tale hole on whiteblock engines, so while they do last a while, they also do fail.
This pic was taken when I did the cambelt on my '97 S70 T5 a few months ago - you can see the corrosion building up nicely which was causing a small and unnoticed leak.
I change the cambelt tensioner every time. Yes, I've read that these can and do last a long time, but the cambelt, tensioner and idler pulley are all equally critical to the health of your engine - if any one of those fails you're looking at an engine rebuild or replacement, which is enough to send many 850s to the scrappers.
I've had two aux belt tensioners start to fail (ie growl) so I'd be tempted to change that as well - but as thats pretty straightforward to swap out, you could leave it be if its running ok and exhibiting no play.