Purchased used, how should I check to see if I should add timing belt or tensioner to my planned maintenance?

 

visual inspection of the belt.

getting records from the previous owner, it should be changed every 60K miles. I would change the tensioner with each belt change. the water pump is usually good for 2-3 timing belts. so if its showing no signs of dripping from the weep hole, leave it alone (replacement pumps are often lousier quality than the older ones).

spark plugs, cap/rotor and wires every 15k to 30k. use Bosch cap+rotor, use copper not platinum or iridium plugs, use bougicord wire sets.

oil change every 5k miles. coolant change every 3 years if you use Zerex G05 or Volvo brand coolant. if you use generic green stuff, change it annually.

ATF every 100k miles or 3 years. use a quality Dexron III, like Castrol Transmax Multivehicle.

 

Change the tranny fluid every 50K.
The water pumps will likely leak from the top "sealing ring" much before the weeping hole, use Volvo part to help that.
The green coolant is OK for 3 years...
The Bougicord wires last much longer than 30k miles. The plugs, use NGK, not Bosch...
Change the power steering fluid, should be clear, not grey or black.