Every vehicle with an an EGR is going to require a replacement at some point, just a case of how soon and how often.
I‘ve got a 180ps CFCA fully converted camper, the torque is nice to have as it reduces gear changes but I certainly don’t need the power if I want to drive within the law.
A bi-turbo assembly is going to cost way more to replace than a single turbo.
Squeezing more performance out of a relatively small TDI is no doubt going to reduce reliability.
The biggest cost these days and almost certainly looking to future is fuel, I’d opt for the best fuel economy without wringing the t*ts off the engine / gearbox.
From what I’ve read the 150ps, 6 speed is ample for a campervan.