We've got a good relationship with the dealer - they inspect our 960 yearly looking for things that are going on it... Usually they turn up with nothing... Spent a few hundred on getting its A/C system going this year (fracturing pipe leading into the evap unit)... Other then that they've found a leaking power steering pump (replaced the whole rack and pump assembly... they said they could try to repair it, or replace the whole unit... we opted for the whole thing) we noticed our rad was leaking, and got them to change it out... then a few years ago leaking slave cylinder in the rear disk brake caliper - replaced caliper... and whole new exhaust system (I forget why.... I think my father wanted a full stainless steel unit when he decided to keep the car more then 4 years).
Other then that nothing has gone wrong (knock on wood) just tune ups... and a new "used" radio this year.
960's don't seem overly complicated... The climate control system would probably scare me if I took it apart, but it works fine so we're not going to go there.... but they used that system for years on the 760, and for years after on the 850/S70's... well it looks the same, controls wise.Power seats you could probably get from a later model Volvo.
Radio's are aplenty, I found, at scrapyeards - all those 850's on the market that got into accidents share the radio with the 960 :-).