I've got a 3.2 manual Q4. The whole drivetrain is fairly bulletproof. I've had mine for 3 yrs without a single fault, and as its a petrol there's no need to check DPF/EGR etc. Unlike the old 156 there's a timing chain instead of a belt, so maintenance is pretty simple too. I've only heard good things about this model (apart from fuel consumption, but you've already noted that).
The usual checks apply as per any 159 - check for uneven tyre wear, windscreen creak etc, and a regular service history with oil changes. Worth getting an expert to take a look too (or plug in and read the error codes yourself).
A fairly common failure on the 3.2 is the oil temperature sensor. If its faulty then your oil temp gauge won't move. A minor fault, but fixing it is a nightmare - apparently an engine out job (I got quoted 10 hrs labour) as the sensor is in the bottom of the sump and hard to get to.
Its a great car though, still puts a huge smile on my face every time i drive it.