As long as the door is of the correct size, the hardware can handle it, and it is not warped or will not warp over time it should be fine.

Keep a backup new door in the garage.

Exactly as Mark said. There is nothing different really using an old door versus a new door- an old door might be thicker or heavier but you can buy track of varying weights and the wall will be framed accordingly. If the wall has already been framed for a standard door slab you may be in trouble. I just did this for a bedroom door in my camp with a turn of the century door but it was an equivalent thickness to modern doors so standard track was fine.

Interesting, and beautiful doors. One of my clients bought a house and found doors like that in the basement. We had them painted to blend in with her dining room and hung them on the wall as featured artwork. So if you can't use them as functional doors, you can always use them as art.

Start with, did you build the wall as a 2x6 wall instead of a 2x4 one? That's not where it ends though.

2x6