The miles are not super important, but there is a relationship between higher miles and things going wrong. It's just the nature of the beast. On a 100, for instance, the motor will likely be in perfect condition at 150k, but you're still 30k away from a timing belt, will likely need a starter, 1/2 shafts etc. Higher miles will mean more expense keeping the truck running. That's the trade off.
Automatic transmissions have a finite life span no matter how well cared for, and a 200k example is closer to the end than a 60k one. This is normal and expected. As long as the prospect of a possible rebuild doesn't scare you, drive on. Not to say the tranny isn't good, it's great actually for an auto, but it will need a rebuild between 200k and 300k if not before.
A higher mileage truck will also require more "tinkering" to keep it on the road. Examples-my FJ60 has 230k miles and I would not hesitate to drive it 3000 miles tomorrow, but rarely does a week go by without some small service-brake pads one week, clutch adjustment the next, wheel bearing repack the next. My FJ62 runs and drives like new, but the expansion valve in the AC system went down this week (hey it was only 105F). 20 years of nearly full time use was enough for it. This kind of little stuff is normal.
Anyway, a higher mileage truck will need more service. That's one of the reasons the're cheaper! As long as you plan for and anticipate this you'll be fine. If you have a choice of makes and models all with 150k on them, which one would you buy? Probably a Toyota, and probably a Land Cruiser. But if you had a Land Cruiser with 50k and a Land Cruiser with 150k and they were the same price and condition, you'd be a fool to take the higher mileage one.