Your immediate problem(s) is to try to get the "check engine" light off IF you want to sell because almost anyone will want that fixed first.
Is there another dealer around? I too find the "multiple part" thing hard to swallow and highly doubt that from my experience with S80s. The car is networked so often one bad part can give false codes for other parts downstream that might not really be bad. Volvo knows this.
As for the sunroof, the rails breaking are common.

"Money pit" ?.....probably will be, especially with higher mileage. I doubt that most newer cars, especially a '99 S80 will last long like older models once did, at least not w/o you spending alot of money on a regular basis. Almost all parts for S80s are big $$ too and if trips to a dealer 100 miles away are needed for software, etc, I'd go for a car where one can get parts and service closer to home. Who needs the hassle?