I think it is awesome that you are researching into ferrets to make sure you get a good start. Too many people buy ferrets thinking they are cute and adorable and then find out that they can be a little more work then they were expecting.
I used to work for the rescue in my area and I have 3 ferrets, and one that has passed away. As far as cages go the bigger the better and it all depends on how often they are going to be in their cage. I have a huge superpet cage for my 3 and they have tons of room to chase each other and play, but really I could have gotten smaller as they are not in it all that often. Good rule of thumb is 3 feet wide by 2 feet deep by 2 feet high per ferret. Hammocks are definately a good thing, but it can take them some time to warm up to them if they have not used them before. I have 3 hammocks in my cage as well as blankets and old shirts and hoodies in my ferrets cage for them.
As far as neutering/spaying that is definately a must!! In males it will help with behavior and with females they need to be spayed if they are not used as breeders because females will go into heat and stay in heat until they are bred. That can cause something called aplastic anemia and will eventually lead to your ferrets death. Descenting is more of a personal choice and most places that sell ferrets will have them descented already. A ferrets that is not descented will be able to "poof" which means they can sort of spray like a skunk, but it is not as bad as a skunk and goes away pretty quickly.
Two of my ferrets are rescue ferrets and I could not be happier with them. Usually most rescues have foster homes instead of shelters, which means each ferret is getting more personalized attention and if they have any behavior issues they are worked with. My one female was horribly abused and she was terrified when I brought her home, but with some love and patience she came around and is now such a sweet girl. My little boy I got from the rescue was 10 months old so he was still a happy healthy little baby. I would definately say to adopt from a rescue if you have the chance....you know then that you are helping a poor little ferret that someone else did not want.
There is a great book called Ferrets For Dummies and I reccomend that anyone even thinking of getting a ferret should read it. It is excellent and gives you all the information you need to know to have healthy and happy ferrets. They can be expensive when it comes to health concerns so that is something that you need to be ready for and have some money set aside just got the little guys. Adrenal disease is a common problem and surgeries can start at like 600 or more.
But I will say they are rewarding pets and very loving and a whole lot of fun. I have had many animals in my home and one that will always be present in my life is ferrets.