First, my take on gasoline. Yes, use 91 octane IF you have a high pressure turbo and tend to use the turbo a lot and high rpms. 87 octane is fine for the rest of us, unless it is very hot and lots of long up hill drives. 89 octane usually has more cleaners in the gasoline and will keep the valves and intake cleaner.

For the S60, newer is always better. I would have a newer high mileage car than an older low mileage car. These engines were meant to be driven and not just to the grocery store and back or once a week to the church. Low mileage coupled with regular engine oil equals sludge and PCV problems. AND, most dealers do not use synthetic oil, it is too expensive for them.

Check the Carfax closely and examine the car for body work. Many dealers, not just Volvo, will buy a car from auction that has a dented door/fender and have it repaired and that will not show up on Carfax.

C30s are no longer made, and parts are going to be difficult.
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Keeping it running is better than buying new