I haven't done a belt change on a 960 but I've done quite a few on whiteblocks in 850s/x70s and they're a bit of a PITA... Access is quite tight between the inner wing and the end of the engine - this won't be as bad on a 960 but I'm sure there will be restricted access caused by the radiator etc.
With that in mind I try to ensure that I don't have to go back and do it again in a couple of months/years time, so I change:
- timing belt
- idler pulley
- tensioner
- water pump only if it is showing signs of leakage or is noisy when rotated
Bear in mind that some tools are highly recommended when doing this job. The square headed auxiliary belt tensioner release tool is very useful, a top quality torx socket to release the idler pulley is critical in a transverse engined car, and the cam locking tool. I tried to do the belt change without the cam locking tool once when a few people said you didn't need it... I now also have the camshaft alignment tool
That cost me £100+ and a 2 day delay IIRC.
I use only Volvo or OEM parts for this job, but I have previously used a Continental belt once without issue.
Good luck!