I was in a similar situation to you when I bought my GT Blackline last year. I'm not mechanically minded so what I would suggest is THOROUGHLY checking through the paperwork. How many owners? Where has it been serviced? Have all the service schedules been met? Are there receipts for any other work done to the car? If you get the reg or VIN and call Alfa they will tell you if any work has been carried out under warranty on the car.
If the car has been serviced at the same place since new then give them a bell, I did this and the dealer was surprisingly helpful, pretty much running through the car's service history with me, telling me that the previous owner was a nice respectable fella and had spared no expense on the car!
Checking through all this will give you a very good idea how well the car has been looked after if, like me, you don't know what to look for under the bonnet.