A thread file with I.D capabilities will do the trick and not cost all that much. But with that being said a new nut will probably cost less. A decent set of thread files in the toolbox is never a bad thing.
I was thinking the same about chasing threads. I know you are needing a nut but then I saw you are looking for a whole spindle so I got confused. I don't know how bad your threads are or where on the spindle they are damaged. In the past I have used a die grinder and flattened the damaged area so I can thread the nut past that area on the spindle.
Don't know if that will work for you. Does that make sense??
So far I used a thread file and got it some what done and where the bad spots are I filed them down and so far the nut has went on up to that spot so we will see