I have access to at least 20 solid oak boards from an old building that was on my father's farm.
They are 12" wide, 3" thick, and most are 14' long, and I am thinking of making a dining room floor out of them.

Years ago, I had some old hardwood floors that didn't have good finishes on them, and at that time, was glad to get them covered with carpet.
Times have certainly changed, but I am still leary of getting a wood gfloor and the upkeep it involves.

So I was hoping for feedback that would help me make up my mind. And maybe now there are much better finishes available, so it would be more durable than those old floors. This floor would get a definite traffic pattern on it.

I'm cconfused on whether to have the job done at all, and if I did, what width the boards should be milled???

You might be better served by posting this question under one of the other subtopics; either "What Hardwood Is Best For Me?" or "Hardwood Floor Finishing" might get better results.

The custom milling costs will probably end up costing nearly as much as just buying some flooring already milled. So unless there is sentimental value to that wood, I'd forget it based on economics. But if you really want those boards made into flooring, contact local lumber mills and found out what they would charge. As for the size, I'd go with a 3/4" thick, tongue and groove plank no wider than 5". Good luck.