Would a finished basement be a good place to house some ferrets? If so, a  dehumidifier is needed...But my basement is very nice, (carpet every where,  a nicely sized area for ferrets, and closed off spaces from dangers)

EDIT: How can I add ventilation to my basement? I has a slight "musk/mold" smell, and I know ferret smell doesn't do well in places with little ventilation...I do know that putting ferrets on a healthy diet works well. If I added a dehumidifier along with a air purifier, would this help clear the smell?

Do you spend much time down there?  It would be good because it's dark, as long as they don't get neglected because they have a separate space

THE DOOKS OF HAZARD

As long as the basement is completely finished and ferret proofed it should be great. If your basement doesn't have central air could you try a small window unit? If not try one of those air filtering column fans. Costco usually has them for a good price. 16 hours of darkness helps ferrets produce melatonin and fight off adrenal so the darkness every day would be awesome for them. Another idea could be to baby gate the basement door up the steps and point a fan up it to keep air flow going.