Is it the 1.9 or 2.4 diesel you are looking at?
At that miles you are in the lap of the gods to some extent, so you just have to use your instincts and judge the general condition of the car, look at its maintainance record, and see how it drives, steers and brakes. If properly maintained the 2.4JTD should be fine at that sort of miles. Cam belts should be changed ideally before 60,000, and a water pump is a must on a 1.9 at the same time, and advisable on a 2.4, as I understand its the same pump on both engines. Don't forget the turbo as well..expensive, and quite honestly at 150,000 miles its something you ought to budget for. They don't last forever.
I'd also advise you take a jolly good look at the condition of as many suspension bushes as you can for creaks, rattles, and disintegration. Also the 156 has the habit of the roll bar shifting on its mounts. Its not a cheap job, but my Indy told us ours was not in bad shape even though it had shifted slightly. He said a new one would do just the same..
You'l like it though, I guarantee that