Moving to a 159 sounds like a great idea!
The cars are very well put together (I run a 156 and 159 at the moment, so I can compare the two on a daily basis) and a real step forward in build, quality and safety terms.
The 2.4 diesel is not that economical in the 159. I do 700 miles a week at the moment and I'm getting high 30's, sometimes nudging 40mpg. The 1.9 diesel in the 159 is not going to match the best in the class, but it should net you mid 40's with normal driving, so it's a step ahead of the 2.4 if economy is the key.
There's not that much to watch out for on the 159. Electrical gremlins are not unknown but they certainly aren't a feature of the car. You really want to be looking at the steering rack, the wheel alignment (avoid heavy tyre wear), the EGR and DPF. Both the latter issues are well covered on this board, and the wheel alignment is a simple case of having it correctly adjusted.
An earlier 159 can be had for not a lot of cash nowadays, so hopefully you can stay out of the Astra!