1) Do they ruin furniture like cats?
They have been known to dig at carpets, which can damage them. Keep their nails trimmed.
2) Do you have to keep them in a cage?
Some people don't, but if you don't have a cage for them they need to be kept in a ferretproof area. If you go with a cage, get a large one. They will need to be let out to play every day. Ferrets are not like hampsters, they need room.
3) Do they make big poopy messes?
Yes, yes, yes, yes, and yes. Did I mention yes? The pile is small and not as smelly as dog or cat, but the volume output is large.
4) How often do you have to feed them?
Keep a bowl full. They'll eat when they're hungry.
5) Do you have to keep them on a leash when you go outside?
I would advise yes. You must also make sure they're legal in your area.
These are high maintenance animals. As for the smell, they can be descented. there are also dry powder shampoos that improve the smell. Regular bedding changing does wonders as well. Like puppies and kittens, they need to be trained not to bite.
Ferrets are theives, they keep a stash. If you are missing something, check their stash.
Ferrets do best in pairs. Single ferret households need to make sure their ferret gets a lot of attention, as the ferrets will get lonely.
Because of poor breeding, ferrets have a high rate of cancer. They live from 6-10 years on average. Be careful with distemper shots, a lot of ferrets have had bad allergic reactions to one.