All the diesels in the 156, GT and 159 are essentially the same engine and use the same waterpump (the weak point). There is much debate on here about what interval to go for but probably the consensus is fours years. As said above, its the water pump that usually fails so this must be changed (despite what Alfa say in the servive schedule). Thus, regardless of what you go for you need to budget for a belt etc change if not done in the last 4 years. The DMF and clutch are also expensive to change. Thus, walk away if there is any vibration pulling away or when you boot it at low rpm.
Why not a late 156 if reliability is your main concern? The 156 came out best of all Alfas on a recent survey. The 156 diesels don't have the DPF and swirls valves like the 159 and late GTs have. Also, the 159s seem to have a lot of gearbox failures in the diesels. The 159 is a prettier and better built car but is much heavier and notably less economical than the GT/156 range. I was torn 2 years ago but am now glad I didn't go for an early 159. That said, any Alfa is certainly going to cost you more to maintain than a Mondeo diesel!