There are plenty of recent posts regarding common failures in our TDI Passat's at this age and mileage.
I would check for oil leaks, such as sweating or leaking valve cover gasket. You may see oil pooling in injector wells or oil stains on the exhaust manifold. Valve cover gasket is probably first to need attention.
Second, check the oil cooler. But in my experience, it is usually the coolant side that bursts and usually from perfect to leaking with little warning. Be prepared to deal with it when it springs a leak, coolant leak will be significant.
Third, despite being below 130k, you may consider timing belt replacement. I did mine at 120k, but some do it even earlier especially due to age. There, you will find camshaft or crankshaft seals, which may have started to sweat or leak. Those should be replaced with every TB change in my opinion, since you're already there. The coolant pump will also likely show signs of leaking as well. Look for pink stains and check the water pump for dark leaks from oil hole - could be a sign of bearing being compromised. There was also a mention of oil galley plug leaking in the same area.
Basically, a good pre-purchase inspection will be needed. You could inspect valve cover yourself with a flashlight and a step stool. Get the car on the lift, inspect oil cooler area and underside for leaks. The service center can easily remove the TB cover and harmonic balancer and inspect for any leaks around cam and crank pulleys. I could see the dark oily residue around the crank pulley and I knew that seal was leaking. Make sure it was well maintained, I did my own maintenance since "free maintenance" period, I documented in the maintenance book, including copies of receipts. If I were to sell mine, the buyer would receive my proof of maintenance for each 10k period. I don't plan on selling though.
Cars of this age, look for the following things going out - door latch actuators (sometimes they start making squeaky noises and sometimes they quit locking and unlocking), the AC expansion valve starts making weird sounds when it is time to replace (after about 15 mins driving, it sounds like a hiss behind the dash - AC works well, just the hissing can get annoying if you have good ears), many folks report Chinese AC blend actuators can get worn. I have SEL with automatic climate control, not sure if SE trim would be affected. Sometimes better to have manual controls than these that wear out so quickly.
Yes, your heater core will clog. Heater cores are sold for $15 each, they used to be close to $70 in the past. It is a very simple swap. I have done it and I have spare heater cores as my maintenance items.
Your coolant will degrade quickly, within a year it will change from pink to orange. Preventative coolant changes are a must. Or a tune for your vehicle to prevent cooking your coolant. Check for how long diesel gate warranty is active. If not long, tune is probably the first thing to do. Then do all these coolant flushes so hope the new fluid will last a lot longer.
Troubles with some having HPFP and turbo failures. I am prepared, but hopeful not to see either of two. Use trustworthy fuel stations for fuel to avoid contamination. Turbo failing will usually show symptoms before shaft snaps. I am at 130k on original turbo and HPFP.
Look, we have a strong BMW diesel community. If. You're looking for space, Passat is hard to beat. If you're looking for a diesel sedan, join us in 328d forums. Solid cars. I have a 2018 328d with 93k miles and 4 cyl N47 engine. And if you need something bigger, look at 2018 BMW 540d with modern B47 engine. Some of these are sold with very low mileage, though not many 5 series 2018 were sold compared to commmunity of us 3 series diesels. Turbos solid on ours, no issues. No issues with coolant degradation either. So heater cores are still original. BMW was already running updated HPFP, but some of us had 2 recalls - improved EGR cooler and more beefed up HPFP recall. Those can also be tuned very successfully and are a handful of years newer than a Passat. I think carfax gives my 328d a $19,000 history based private party value. But it has more safety features, such as blind spot monitoring and head up display is convenient. You have a responsive head unit than Passat. You can get a better trim with parking sensors, speed limit readers, automatic braking. Basically a more modern and feature rich car for a few grand more. And these are made in Germany with more than half German made components (vs. Mexico and China for Passat).
In short, do I love the Passat? I do. Will I make it run forever? I will try. Would I trade my 2018 328d for a Passat? I miss the space in the back and the trunk, but I would not. If I could go back to 2018, would I have done anything differently? Yes, I would have special ordered a 2018 540d, it would have given my family more room.
I am still hopeful diesels will return. For now, EV is the hype and the biggest lie of the environmental generation.