1.9 diesels have quite a weak gearbox and clutch - repairs can be expensive. Read some of the threads on the Forum. The 2.0 and 2.4 have a stronger box. Post 2008 1.9s can have noisy resonance from front offside, usually blamed on drive shaft joint, but if your car has it, it can be hard to eradicate. Affects Model year 2008 on which have hollow anti roll bars to amplify the noise.
MY2008-on were slightly lighter, have boot opening by pressing badge, electronic "differential", and 1.9 Lussos have Alfa Romeo branded Brembo brake calipers but a smaller disc than 2.4 or Ti.
Steering racks should be checked for unhealthy groans, leaks and the colour of the fluid.
Front subframe rust can be a hidden problem - its not easy to see without taking off the undertray.
Suspension clunks, clonks and rattles shouldn't be there - anything from worn bushes to broken springs are quite common. Just make sure the one you buy doesn't suffer from these problems.
Cambelts should be changed at regular intervals - defintely not more than 72,000 mile intervals - and make sure that water pump is done at the same time.
159 diesels tend not to use any oil - however check it has some in, the dashboard oil level indicator can't always be relied on.
All electrics should work properly.
Check spec vs price. Several variations which can seriously affect value. Ti and Lusso are top specs. Ti has 19 inch wheels (and expensive tyres) Always check alloys for signs of kerbing.
Check inside edges of front tyres, especially on Ti versions as many cars left the factory with the poor tracking.
Add fuel and enjoy.