If you are buying any car at that mileage then you need to see a complete service record and receipts, and you need time to go through them. If the car has been looked after then there is no reason why it should not continue to give good service.
Driven modestly we seem to be getting 45-50MPG out of our 2.4 (150), so you won't get much more.
The only real advantage of the 2.4 over the 1.9 is the sheer effortlessness of the performance. Its very seductive, and sounds better on the move. It is a lot heavier, but this is only relevent if you like throwing cars around a lot, otherwise you won't notice it hardly.
The 2.4 seems to have the best reliability record of any of the Alfa engines ( brera/159 excepted as their record is too short so far) but it does need the cam belt changing every 60,000m (official 72..but don't go there), turbos do fail from time to time, AFMs fail now and again, and the 156 suspension is not terribly robust so check the bushes are good..front and back..and listen for creaks and sqeaks from the suspension.
Something cropped up the other day on the forum from a guy in Oz..check the door hinge bolts are tight. Its stops rattles and improves the 'tightness' off the car.
As for chipping..well I'd run the car for a month or two before deciding. You might find it has all the power you can usefully use on todays roads..even a 136. On the other hand people say good things about Angel tuning and if you want the extra oomph then thats the place to go, but remember you will be adding stress to a mature engine so reliability might be compromised a little. They will happily answer questions on that score.
Its a great car by the way, and I'm pretty critical so it took me years to embrace the 156 at all.
Except the rear wiper which is slow and pretty crap
Good luck in finding a nice one