Hi Glen,
The 2.7 and 3.0 TDI Allroads both have Tiptronic gearboxes in the automatic versions (a traditional 'torque converter' type with planetary gearing and ECU control). They are generally good strong gearboxes and do not require the oil changing as a serviceable item unlike s-tronic and multitronic (CVT) gearboxes do.
The 2.7 is slightly better on fuel than the 3.0, but only slightly.
There is generally nothing that is failure prone, intake manifold flaps can get play in the support bushes over time, requiring replacement of the intake manifolds (however, this was also a problem with the BMW 3.0 diesel like you currently own, so is not an manufacturer specific issue).
The air suspension bags can get holes in if abused or in other exceptional circumstances, but this is rare, as can any pneumatic suspension. Just check the suspension works fine through lift and lower modes and does not drop once the engine has been switched off for a few minuets. Generally, a well serviced example will be a reliable car.
Tech