Can you really or is it like the T1N Sprinters where yeah you technically can take the longblock to half a mil or more but you'll have replaced everything external to it twice over by then?

It's pretty common for this generation of vehicles, especially diesel. This was before all the environmental things were added, which may be good for pollution, but are terrible for longevity. In parts of Europe everyone has diesels, so mechanics know how to fix them, and wearable parts like injectors and turbos can be easily and cheaply remanufactured.

I have a 2007 Fiat Ducato which has done at least half a million kilometres (the odometer only goes up to 399,999). Only issue is the EGR valve is stuck closed (can't get to it without removing the engine, so not worth fixing), and the body is banged up (ex builders van). Still gets 8l/100km (30mpg) on the highway.