Problem with the active fuel management seems to be the rings in the non-firing cylinders floating around and causing excessive wear in the cylinder walls, leading the related problems. Combine it wih indifferent maintenance / infrequent oil changes and you get all sorts of accelerated wear. Plan on engine repairs, plan on at least an overbore and new pistons, rings etc and disabling the AFM. Or going whole hog and put in a new short block and still disable the AFM.
IIRC there's a factory fix for the excessive wear, but I don't know what the restrictions or qualifications are for it. Might have to be original owner.
When I was in the market for a used Sub I only looked at the -900 series long enough to know they were well out of my budget.
I just flogged mine on a 300mi day trip on Easter Sunday, averaged 75mph and 15.4 mpg, which I'm pretty happy about. 126k mi on it, other than a fresh intermittent knock sensor alarm, it's running solid and strong. I wouldn't be worried about a 'high mileage' Vortec unless you are planning to drive it hard and far and daily. Even then, parts are inexpensive and common. All the better if you can wrench ot it yourself.
