Some obvious things to check have been sorted - radiators tend to corrode so become ineffective anytime from 6yrs old on, oriinal water pump had plastic impeller, know weak point so should have been replaced with one with metal impeller (from I GTA I think?).
Suspension arms and ball joints have bonded rubber bushes which lose bond - not a safety issue but the creaking drives you mad and maybe handling slightly detiorates, mine (front) last between 2 and 4 years not had a problem with rears, but I do have Sport Pack 3 - stiffer suspension and driving over awful potholed roads isn't helping!
Rof only panel not galvanised - check around windscreen corners for corrosion, especially if screen has been replaced.
Check idle control when warm - bouncy or nearly dying revs on petrol engine could be lamda needs change, MAF (airflow) sensor needs clean or change, throttle (idle control) or bypass valve needs cleaning.
Check the oil is kept topped up.
If you do I search I think there is a 156 buyers guide.
Regardless of these frustrating but not too serious issues, they are great cars to drive, still look good after 12 years or so, interiors are classy (but can have a few creaks) and have great engines - my V6 is still fantastic fun.
If you get a car thats been looked after, and they are seriously cheap now for what you get, it will certainly feed the alfa bug in your son!
07 3.2 Q4 SV Brera alfa red --- previously '99 156 2.5 V6 for 9.5 years