As with all cars, check it carefully and purchase only one with all service stamps.
The V6 engines are bombproof and has a wonderful sound track. The 147 bodies have no special problems and the 147 have really good handling. 2006 models has larger brakes (necessary) than early models. I have not heard bad about the transmission.
147s are really drivers cars.
Good luck finding a good one.
Friend that I race Alfa Suds with has a 147 GTA and drives it reasonably hard. He has had no end of trouble with the diff. So far he is onto his 3rd. Dealer has repaired them all under warrantly but have now put the "later" one in which is apparently stronger. Not sure on the finer details
Thanks for your very helpful comments -- I was thinking about a 147 gta second hand but they are still quite expensive so now opting for an Abarth 500c as a substitute.....not the same, certainly engine note but somewhat comparable performance.