I don't have a 140, bot would love to..! I do have 2 Farmall A's, which preceded it a few generations. I believe the weakest link of the whole tractor would be where the front steering assy. bolts to the engine block. I have seen a few of these break. I know what I've done to mine, and what was done to the one we had when I was a kid. Let me just say, they will take quite a beating, so those front ends that broke, had to take a pretty good shot. Maybe like dropping in a big hole, or just plain ran into something at a good clip.
The steering tie rods can get some slop in them, but they are adjustable. And the steering arm, out of the steering box can get some wobble to it, due to it coming loose on the steering shaft out of the box. And the bearing around that shaft, plus the seal, after many, many hours of use.
Other than that, if the one you get is in decent shape, with some good maintenance, it should last as long as you'll need something to cultivate with.