If you have a good Alfa specialist in the area, it isn't a bad idea to buy it from them. Decent specialists don't last long if they sell rubbish and don't back it up. I bought our 159 from our local specialist, it has been spot on, and if the engine fell out tomorrow, I am confident he would look after me.
Engines? 2.4 diesel is the dogs danglies if you want reasonable economy, get rid of the DPF/EGR if you want more economy and power. A well sorted 2.4 is a marvellous thing. If you only want a week end toy, then the 3.2 has to be tried, but is not a sane proposition for a daily driver with juice at £1.40. I don't really see the point of the 2.2 petrol. The 1.9 is a good engine, but the 159 is a bit heavy for it in standard tune - DPF/EGR fix will bring it alive.
Service history is important, and if looking at something with high mileage, check the belts have been done. 72k or 4 years for the 2.4 I think.