"...Should I carry a spare one?..."

Might not be a bad idea. But rather than purchasing an after market unit from the local chain auto parts store I would prefer to grab a couple of OE Volvo coils from a pick-n-pull yard. You could swap them into the car at the next plug change just to make sure you aren't hauling a dead coil around in your trunk. Often times an "old" Volvo part ends up being better than a "new" aftermarket part.

As for what brand of hoses to purchase, I look at it this way - the original Volvo hoses, along with the coils, have lasted nearly 400,000 miles. I'd say that's a testimony as to their quality. But then it's easy for me to sit here and help you spend YOUR money.