If you only do little mileage a diesel will probably more more hassle than it's worth with EGR problems etc. The 2.0 JTS (petrol) is a decent enough car, coil pack problems seem to be the most common issue and don't expect it to be quick or sound particularly inspiring. The 3.2 V6 is a stunner but you will get early 20's MPG (less if you are really going for it) and servicing and parts will be more expensive but you will forget about all of that the minute you start it up.
Just buy carefully because as tempting as a cheap one will be it is best to get one that has had a fair bit of work done of late to make it all as it should be (they are 10 years old and over). I bought one (3.2) at what I thought then was a reasonable price and then had to spend over £2000 due to head gasket failure, new clutch and cambelt. Suspension seems to be an issue with most, mine is fine though and I believe it hasn't been modded but there are loads of modding options to make the suspension better. Eibach, Bilstein, Koni etc. Electrics can be a bit random in them but in a typical Alfa way mine seems to sort itself out when the windows decide to stop opening etc just by turning it off and on again haha!