Tuneup intervals are mentioned in one of the owner's manuals. You can also search the Brickboard archives and FAQ. But:

if you are doing the timing belt, you probably should do the major service: Timing belt, fan belts, plugs, air/oil/fuel filters, oil, coolant maybe even power steering/transmission/brake fluids. Time the engine -- you'll need a timing light. Set the idle. Replace the oxygen sensor (get the generic Bosch one from a local auto parts store). Check your shock absorbers, too.

While in your engine compartment, check wires for frayed/flaking insulation. Check all vacuum lines and fittings -- they quite likely have hidden cracks. I've got my '86 scheduled for replacing all the vacuum lines/fittings this summer.

Your Chilton manual is probably fine for this, but go down to your local library and look through a Hanes manual: perhaps its got some additional information you'd find interesting. Or buy the Bentley manual. Or call 800.25-Volvo and order the correct factory manuals for your car.

You can do even the major service as long as you are careful, follow the procedures, and have the right tools. If you have questions, post them to the Brickboard. We'll help.
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854