Full time Volvo Mechanic/shop owner here. As you can guess, we service and repair a bizillion Volvos with the SI6.
They're fairly straightforward and simple to work on. P3s are fairly robust cars. Suspension stuff can be a little more difficult than the P2s but its nothing to really worry about as it all seems to hold up pretty well.
As far as drivetrain goes I can run down some of the common things I typically deal with:
PCV system: It's all integrated into a breather box on top of the engine. They all go bad at some point. Very easy to change in about 5 minutes.
AC compressor: These are another part that you'll be replacing. They get "lazy" as they get old and will eventually take a while to cool down. It's the solenoid in the compressor that is the issue. We just replace the entire unit now due to them being very easy to replace (about a 15 min job not including evacuating and recharging) and compressors aren't that expensive ($380ish for a OES Valeo unit).
Blower motor: I do a handful of them a year. If it's not working, I always try the resistor first. It's a bit of a bugger to get to but it's separate of the fan assembly so I always start with it first. If you wind up having to do the motor assembly you'll need the special tool. I've got it down to a solid hour or so to replace. It's a solid hour of frustration to some extent but not the end of the world.
Aux belt drive: Volvo says to replace them at 10 years or 150k miles. I like to do it a little sooner. That consists of replacing the belt, idler, and tensioner assembly. Almost always the failure is the pulley on the tensioner. It'll lock up and smoke the belt. OES components are fine, as every aftermarket box I've used has had an OES Litens branded tensioner assembly inside. I think they're the only ones that make the assembly.
RH pillow engine mount and upper torque rod: It's under the coolant tank, these are almost always bad if it's not been replaced already. Once again, cheap and easy. OES will be Corteco. All of the other engine mounts seldom, if ever fail.
Cooling fan: Notorious for the cooling fan module to fail rendering the fan staying on after the car is shut off or no operation of the cooling fan at all. One phillips screw and two 3 connectors and its out. It's located on the fan assembly.
Brake Vacuum pump: Located on the drivers side of the intake cam. These all eventually weep oil. All is you need is a rebuild kit which consists of a couple of O rings and new nipple. You're local dealer will have PLENTY of them in stock lol. It's a very easy 15 minute job.
Alternator Coupler: See this issue all the time. The drive gear on the READ side of the unit that drives the alternator becomes loose with age and slips, rendering the alternator not turning. Most shops that don't know any better just assume the alternator is bad. If you run into this issue, just take a peek down under the intake and see if the coupler is spinning. If not, pull the intake and alternator and remove the bolt holding the coupler to the drive unit and slather some loctite on it and ugga dugga it tight. I don't know why Volvo didn't key this pulley
Transmission: The Tf80s at this year point is pretty much got all of the shortcomings worked out. It's still imperative that you have the change to drive it from a cold start and take a good 30-45 minute test drive. If they have torque converter lockup shudder or flaring, it typically won't show up until a long test drive. A little bit of TQ lockup shudder is nothing to really worry about but it's something worth checking.
They can be a little thirsty on oil, some worse than others. I fill up to 8.5 quarts on them. That typically gets them to the next service interval without triggering the low oil level message.
As far as the READ unit, don't worry about it. In the past decade of working on these things full time, I've never seen one bad.
That's pretty much the lowdown. Everything else is pretty much typical used car things. I will say if you plan on keeping it, look into getting VIDA and DICE unit. It's pretty much a must for any Volvo 1999+ for diagnosis.
If you wind up getting it and have questions, feel free to ask!
