Believe it or not, your problems are typical of many '99, '00 S80s and there isn't just "a computer" but several and too many actually. The car's an overly complex, networked "beasty" (being as nice as I can be realistically).

"Also, we're almost due for the 60,000 mile maintenance and they charge over $600 for this!! Is it REALLY necessary?"

The 60K service is a good idea and it's essential that your serpentine drive belt (NOT timing belt) be replaced at that mileage because if it breaks, it often burns through the t-belt cover, takes out the t-belt too and valves hit pistons (will run over $3,500 to repair). The 60K service isn't even close to $600 at our dealership, more like less than $400 complete but maybe they have a higher labor rate that us? It's 4.5 hours for a T6 S80 by us, 4.0 for a NA 2.9 model plus parts as listed in your maintenance manual check guide.

The ABS problem is probably a bad ABS control unit (common) which then triggers both a check engine light and trans error codes too because they don't see a signal from an ABS wheel speed sensor. For an S80, that control unit costs about $650 now plus labor.

"What more can go wrong?"
Unfortunately LOTS of things, the trans can fail (not unusual), many suspension problems (typical), more electrical problems, I could go on........