Re engines, it really depends on what type of driving you do. The diesel will be great for long drives but will not enjoy short hops to the shops etc (will get DPF and EGR clogging issues, unless the car in question has had these parts removed - cost around £500-£700).
If the car is going to do a lot of short runs (sub 10 miles), then you're better off with a petrol. MPGs may be slightly lower but the petrol engines will not grumble about doing short hops and will be trouble free.
Personally I don't like the weight at the front of the diesel (and the effect on turning), but many love the torque. Also the weight at the front of the newer 2.0JTDM really isn't that bad.
As for maintenance costs, I can tell you from experience that Alfa main dealer costs are significantly cheaper than BMW and Audi main dealer costs. Independent specialists are cheaper still. Parts are cheaper than BMW too. More expensive than a Ford, but they're not exactly pricey. So I would give Alfa a big tick on maintenance costs.
Overall I'd say go for it. If you're looking at BMW and Audi, Alfa will tend to work out cheaper. Just do all the usual checks, like full service history, HPI etc, and just generally use your Spidey Sense as to whether the car has been abused or worshipped! There are cherished ones about, get one when you see it!
Brat Pack no.8