Nice choice, it's a good looking car. Whilst I do not have a 159 I know of someone who does and he says it's a nice car to look at (obviously) and drive. I'm only 5'9 and found the rear of the car to be a little cramped, however it may be fine for a 6 year-old and a baby. The 159 also has a 5-star NCAP safety rating so your children are in a safe place. Boot space is decent in the saloon but the Sportwagon has a good deal more, see what you think from the pictures on the Parkers site: -
Alfa Romeo 159 Sportwagon (06-11) Car Review - Summary | Parkers
See if there is one for sale locally so you can arrange a viewing. As for mileage, a well maintained 1.9JTDm 16v (the only engine I've experienced) should prove reliable. Mine is still solid at 70,000 miles, as you have a GTV you may already be used to weekly oil and fluids checks, just to be on the safe side. Like the petrol engines, the diesels do use oil. Also if you can, check the suspension by pushing down on the front wings, squeaking or knocking could indicate a front wishbone issue.
Check for the usual suspects such as full-service history, timing belt/water pump replacement and ask about any previous repairs. For example, a clutch replacement on the diesel engine can cost the best part of £1000 because the dual-mass flywheel usually has to be replaced too. Parts in general tend to be slightly pricey. You could start a thread asking about Alfa parts suppliers in Northern Ireland. I've had experience with two suppliers which are AlfaWorkshop and Shop4Parts and these guys do post to N. Ireland.
The electronics have proven to be reliable (on my 147 anyway) but check everything works nevertheless. The 159 has a good selection of trims, the Lusso trim is very nice and it is surprisingly hard-wearing.
AlfaWorkshop have good buyers guides which will let you into any known issues with the car and also give you pointers when purchasing. I've detailed a few but the 159 guide on their site is far more comprehensive and should clue you in to any problems. They also have a classified ads section which you may find useful.
Link: Alfa Romeo 159 Buyer's Guide
I hope this is helpful and I wish you the best of luck with your purchase.
147 Sport Q2: Autolusso Remap + Swirl Flap & EGR Delete, Mocal Oil Cooler Kit, JTD Performance FMIC Kit + High Flow Elbow, Wizard Downpipe, PF Pads + 5.1 Fluid, Autolusso Braided Brake Lines, MTech Short-Shifter, Autolusso Silicone Turbo-to-MAF Hose, Refurbed Q2's + Some to add later.