I’m no expert, but after 80k miles and ~7 years of ownership I know my way around them.
There’s many things to consider when buying. I had to compromise. I wanted, spec wise, DAB and B&O. Had to pass on B&O. Was just too rare, and I wasn’t going to buy the wrong car the right spec.
I focused on finding a car that everything worked on, that had been well maintained. Mine was a one owner.
The cars are complex and the more spec, the more there is, generally, to go wrong.
In a nutshell the EPBs are the Achilles heel of the D3. I’ve replaced both electromechanical callipers once and will likely have to replace the near side again imminently.
More import is the air suspension. I’ve had a level sensor fail, which raised the back, lowered the front and the bumper caught on a kerb.
Like I said they are complex and ageing. Still a lovely old barge tho. Big, quiet (double glazed), not slow (even the 3.0 TDi) and just a nice place to be.
Keyless entry is another common fault. The micro switches in the door handles themselves fail and cannot be serviced. So it’s replacement only, it’s a fiddly job and you need them colour coded.
They are very hard on their batteries too, as standard. Lots of owners find their batteries flat after leaving the car for a week or two.
I can’t see me parting with mine TBH, the whole family love it. Hence me getting a second car to have some fun with, rather than get rid.