As a general rule you get an ideal of how suitable a car is by its recent past. If say for the last 10 k miles the car has not developed any serious issues. It will stand a much better chance of doing the next ten k miles without them.

Now if serious items that imobilize the car on a monthly average for example are occuring. The reliability is in question. You do not really want that car on a long trip. You kind of develop a sense of how reliable a car is likely to be. Sure there can still be unexpected issues crop up anyways. Just less probability if the car usually never normally has them.

If you take it easy on the old car helps as well. You also should not go the ten k on the oil. These engines accumulate soot in the oil that acts as an abrasive. You might aquire even more than normal with a heavily laden car working hard.

Have an enjoyable trip. Low funds where common when most of us where young as well. Most still did things in spite of it.