You know, I own a few vintage pens and the thing that is the last thing to worry about I find is the nib! I also have a Parker Duofold made in 1928. It has the original nib and the only thing that has been done to it since it was made was a new ink sac replacement! and I write with it every day! in fact, it could be said that the older pens are much better made and more reliable, remember, back then there was no alternative to write with so fountain pens were huge business and it was a very competitive industry so pens had to be hardwearing and reliable for the makers to last.
That did not mean they had to be expensive either, look at Esterbrooks with their interchangeable nibs, not expensive pens and amongst pen collectors today very desirable.
With vintage always check to see if it has been serviced, if so and they usually are, then you will probably be good to go for another twenty/thirty years before an ink sac needs replacing! For the Aerometric 51's, well they are all but bullet proof! I recently got a 51 special from ebay, I won it for $25 as it was being sold as is. Once it arrived I flushed it in warm soapy water till all the old ink was gone, then clean water and then filled it with ink. The pen writes like a dream and that was all that was needed doing, very rare a sac needs replacing on those models and the one I got was made over 55 years ago!