The seller says it just started misfiring & I don't know how much. I don't know what a DTC is, but there are 177,000 miles on it.
A DTC is a Diagnostic Trouble Code. I thought you had the car at the shop for the problem and they told you the timing belt was causing the misfire.
Is the check engine light on? Does it flash?
We don't have the car. We're looking online, but don't know whether this type of repair is affordable or not.
Okay, well first of all, the timing belt is due every 105,000 miles on that engine. Hopefully, the belt has been done. If not, it is truely running on borrowed time. If the belt breaks, severe engine damage is likely to occur.
As for the belt causing a misfire, it is not likely. If it was recently replaced and they installed it incorrectly, it can cause a misfire but it more likely wouldn't start.
Without having some trouble codes to fault trace, I can't tell you for certain what is causing the misfire. If the spark plugs have not been changed in a while (they are due every 60,000 miles on that engine), I would start there. That is by far the most common cause of a misfire. Next most likely is a bad ignition coil (or more than one coil). But since you have 6 coils, it will be hard to narrow it down without trouble codes. If there is a code for a misfire in a particular cylinder, I usually swap that coil with one from a cylinder that is not throwing a misfire code. Then I clear the codes and drive the car. If the code moves to the other cylinder, the coil is the cause.
Those are your two most likely causes. If I remember correctly, coils are about $100 each and plugs are about $65 for a set. But like I said, without codes, it is impossible to tell what exactly is causing it. It could be a number of other things (Mass Air Flow sensor, CVVT hub, camshaft reset valve, or a number of mechanical problems).
Honestly, the 99 S80 T6 isn't the best car. The transmission has had a lot of problems, though mostly in the XC90. Depending on how it has been cared for, I wouldn't expect that car to last much longer based on that mileage. I am a Volvo technician so I can get parts at a large discount and perform the labor for free and I don't think I would buy a 99 S80 unless it had significantly lower mileage on it or I could get it for $500-$1000.
If you are looking to spend $2000-$2500 on a Volvo, I would recommend a 95-87 850. The engine is more reliable, as is the rest of the car. The problem is they are kinda ugly, being much more boxy than the S80. If you really want an S80, go with a 5 cylinder (04-07 I believe).