2015 and 2016 4cyl engines are prone to catastrophic oil consumption and engine failure. After 2017, all XC60s used an updated version of the 4cyl engine which is very robust, particularly in the T5 model without the supercharger. The 4cyl cars have an 8-speed automatic and auto stop/start, which helps them achieve significantly higher fuel economy than the other options. I would avoid any 4cyl produced before late 2016.
A 5cyl XC60 is also at risk for oil consumption, but not prone to cylinder wall damage like the 4cyl. The 5cyl requires especially careful belt maintenance to avoid bent valves, but it is a very, very solid engine with oodles of low-end torque. Maintenance is everything with this engine in particular. In my opinion, it's the most characterful and the best-sounding of all P3 motors. If you do end up with an oil burner, there are some "at-home remedies" you can try to unclog the piston rings, which did wonders for mine. I would buy another 5cyl Volvo with this engine, even knowing the risk of oil burn, because I have really enjoyed mine so far.
The 6cyl cars are very robust and generally regarded as the toughest of the bunch. Sidenote – they're commonly referred to as a V6, but they're not. They're unusual in the sense that they are a transversely-mounted inline six. The 3.2 is very smooth and willing to rev, but around town, it's just not in the mood. A T6 is much better in that regard. Both engines are considered desirable for their refinement and their longevity, but fuel economy is pretty poor. They can also burn oil on occasion, but like the 5 cylinder, they aren't in the habit of blowing themselves up. The 3.2 and 3.0T6 also a timing-chain engine, which some people are really fond of. At 150k, you will still need to change the auxiliary belt.
All 3 of these engines (and most Volvos, actually) are known to eat their PCV valves at higher mileage, especially if they've been driven for a long time with dirty oil. This shouldn't scare you too much, it's just something to be aware of. They require full synthetic oil changes at or before the manufacturer interval. When in doubt, a 5000mi oil change interval is wise for a modern engine.
I don't know of cooling issues with any of these cars. Last I knew, Volvo uses (or used) a Ford (?) testing center in Arizona where all their models are ruthlessly evaluated for durability. I've also heard that Volvos generally have a lot of cooling and thermal management built in – so I wouldn't worry about overheating. A coolant flush, a fresh water pump, and regular maintenance are your best defense against hot temps in any car.
In the US market I believe the engine choices were as follows:
2009-15 3.2 FWD/AWD, 240hp/236tq
2009-16 3.0 T6 AWD (286 or 300hp depending on year)
2014-15 2.5 T5 AWD, 250hp/266tq
2015-18 2.0 T5 FWD/AWD (avoid 2015-16), 240hp/258tq
2016-18 2.0 T6 FWD/AWD (avoid 2015-16), 316hp/295tq
Note that only the 2.0 cars will have the updated 8-speed gearbox which helps fuel economy. The other cars use a robust but sometimes clunky 6-speed. Both transmissions are Aisin units and generally very solid.
You'll also notice that 2015 and 2016 got very confusing. These were transitional years for the P3 platform as Volvo added 4-cylinder engines to its lineup. In 2016, for example, you could get your XC60 with 4, 5, or 6 cylinders. In a futile attempt to reduce confusion, Volvo badged the 2016 6-cylinder T6 AWD as the 3.0 T6 before it went away entirely for 2017.