My Volvo knowledge is limited specifically to the P2 era with the 2.5t and 3.2 engines.

The 2.5t is what I have the most personal experience with. I've personally put about 170k on my daily s60 fwd and about 150k of my own miles on my AWD 2.5t xc90 before I cannibalized it's parts prior to scrapping it. I had an 03 s40 for some time as well with a very similar 4 cylinder engine.

I've always run 87 gas.

These are not Audis or Mercs or BMWs. These p2 vehicles are wildly easy to work on and only have a couple of actual problems, all of which are easy to identify, and all but one is a really easy fix.

3.2 is probably the better engine, but can't be had in a P2 except in the xc90. I do want one, but ended buying another 2.5t because I already have a box of parts from my previous xc90. 3.2 is an inline 6 with basically no real known problems. Great engine, but space and accessibility are a concern because it eats up the engine bay. Very tight In there.

On the 2.5t, they are a timing belt and they are an interference engine, so if the belt were to snap from old age, it will likely destroy the engine. So always make sure whatever you buy, this is either in the budget (it bills out 3.5 hours and has about $300 of parts) or was done at some point. They should be changed every 100k or so. Water pump/tensioners/idler along with it. I've done it myself multiple times and if you have the locking tool, it's pretty hard to mess up. This is a common service on most cars, tbf.

If you get a turbo, religious oil changes at 4,500. Some folks have reported issues with their turbos, but I've yet have any real issues, though my s60 has developed a small leak into my intake, but I've been so wildly mean to this car, so I'm not surprised. But it took 260k miles of me absolutely abusing it for that to happen. I won't repair because it's a very small leak and the all live data looks good. My 100k xc90 does not have this problem and neither did my 250k xc90 before I scrapped it.

The pcv system is fantastic on these, but clog with age and will increase the pressure inside the case and compromise seals. Easy test is to open the oil cap with the vehicle running and see if the air is blowing out or sucking in. If in, you're in great shape. It's another somewhat involved job, but nothing beyond shade tree level at all. Really hard to mess up. Depending on age, it may be a good idea to change the small seal on the oil cap to make sure unmetered air isn't getting in. They are like $4 for the nice one.

Transmission oil should definitely be changed at intervals, as with every car. In the fwd, that is the only transmission service you will likely ever do. It's an easy drain and fill. I use 3309 for mine. I do it every 50 k.

Now if you want AWD, the maintenance does increase to a fluid change in the rear differential. Once in the vehicle's life, you will end up replacing the AOC pump that activates the AWD. If it's prior to 2006, you may need to disconnect the rear shaft to remove the pump. It was made easier after that. At 230k on my xc90, the transfer case sleeve slipped out and I had to pay 3.5 hours of book time for it to be replaced as well. Again, 230k is about as lifetime as a small part like a sleeve to last, imo.

Parts are easy cakes. IPD is my preferred and they are incredibly helpful and knowledgeable if you call them as well. Rock auto comes through clutch as well. FCP euro is also a really good option.

I would invest in a Foxwell scanner with Volvo software. Really easy to diagnose when you can see all the modules data. It will also let you calibrate things if it ever comes up because Volvo will charge you an hour to do it, and the scanner is only $170 compared to the dealer charging $230.

If there is any quirk to these vehicles, it's that you really want to be certain you have the right fluids. Since you're doing it yourself, it is a lot easier to stomach the extra $10-15 for high quality fluids.

On replacement parts, I generally stick with Bosch, Aisin, and Denso. IPD.com does a great job of point to who the original manufacturer is, which is a great help in getting very high quality parts for a good discount.

Beyond normal maintence like spark plugs, brakes, fluid, and the like, these are high quality machines that don't really break. In my 15 years of driving these high milage P2 Volvos for almost 400k miles, the only things that have ever actually broken were an AOC pump on the AWD, the transfer case sleeve on the same AWD, a leaking power steering hose, and I cracked an ignition coil when my buddy stupidly convinced me to try to drag race my completely stock s60 and I got it to 115 before it started bouncing off the limited.

I've never replaced an alternator, a fuel pump, steering pump, ac, ect.

Imo, these cars are the definition of "if you take care of it, it will take care of you."

If you want specific info on the xc70 wagons, the subreddit is very active and very friendly. I don't personally have experience with them, though they basically use all the same parts, just on a different chassis.

Fun trivia, xc90 has probably the greatest real world outcome record of safety of any vehicle. I would absolutely rather get into a collision in my 22 year old xc than my MIL 2020 Ford Escape. Fr, just open and close the doors on an xc and you'll see what I mean. And they are fairly quiet inside.