I have three ferrets. Here are my top three things:
1) They are poop machines, and are difficult to get litter trained. You'll have to spend a great amount of time on this and realize that some ferrets will never be completely litter trained. My three are, thankfully -- but I spent a ton of time on them.
2) Ferret proofing a room is a must -- it's not as hard as people say, I just think people are very ignorant and not attentive to detail. Just keep things high up off the floor on tables/counters/etc., make sure there's no small holes they can escape from along the floor/wall, and make sure there's nothing they can climb that's in the vicinity(1-2 feet) of your tables and things that are high up. Not much to it.
3) They will have vet complications and are expensive. I went to the vet the other day for one of my ferrets who had some very questionable poop and kept producing a lot of ear wax even though I'd clean his ears every three days. It cost $189 dollars for the visit(then again, I went to a very good vet in a very rich area -- I trust him) for just one ferret. Thankfully, I can afford whatever costs the vet will ever need so I have the joy of providing a home to these awesome animals. I had the $$ before I got the animals. So many people give up ferrets because they just don't understand what they're getting into when they get them.
So yeah, lots of poop, secure the area, and know that vets are expensive and that they will have problems. Good luck.