If the camstack of the Viking 6440 haven't been replaced in the last few years, it will for sure have to be. I understand it was a common flaw of this model. It crack and end up not working. I have repair a few vintage sewing machine (Bernina, Singer) but I wasn't able to fix the Viking I got for cheap myself. Jut opening it was hard. If you buy one and end up having to bring it to a repairshop, it will be costly because they take more time than most to repair.
If you want to go vintage, why not looking at a Singer 15-91? If you don't mind the modern feature like thread cutting, needle up and down, you may be happy with it.