Volvo uses a decelerometer in the center console to trigger
the air bags. I'd have this all done by a dealer who is familiar
with all the details and has all of the manuals and can test all
of the components (the seat belts also have tensioner devices that
may need replacement and the side air bag system should also be
considered). You may also need special equipment to test the system
and reset it so you don't have a dash light constantly on. The seat
belts should also be replaced after an accident(they stretch).

I'm actually curious if a dealer would sell you the air bags.
Have you looked into that?