Auto box - is it M'tronic or Tiptronic? Latter is much more 'reliable' I think is the right terminology but it depends on no. of factors...
Engine - personal preference is larger engines but 2.0TDI for instance is also good enough so long as it is late 2007 onwards (e.g. BRE engine code)
Challenges - you'll have some such as DPF - EGR, flywheel, potential oil pump/chain 'concerns' etc... but the general advice so long as it is adequately maintained and backed up by service history (with receipts to proof) you should be just fine.
They are great cars to drive on longer journeys and that coupled with higher octane diesel (BP's ultimate or Shell's VPower/Nitro) you'll enjoy it... hope that helps and i'm sure other members will add more to the above.
Personal
