99% of ferrets will be hit or miss with the litter box all through there life. It's a very rare occasion where a ferret is TRUELY litter trained, no matter how hard your efforts are.
I'm sorry she passed away, ferrets are said to live up to 10 years or older, but it seems about only half of them make it that far, we've noticed it to be more like 6-8 years.
Keij's already 3 1/2 - 4 years. I got him as an adult from a pet shop for $40.00 (unusual price) a year ago.
Most rescues adopt them out for about $80.00 I have noticed. But for the ferret, which is normally $100 a peice, the cage which for a GOOD one is about $150.00 + easily, not to mention everything you need to put IN the cage, to ferret proof your home ( or at least one play room ) and then the immediate trip to the vet, can add up to a lot.
Some ferrets small strong (specially if their not descented) and others not, it really depends on how the ferret is naturally (disregaurding any illness's), what you feed them, how you groom them and how well you take care of their cage/bedding. Keiji to me smells like Lemon Dust.
Though I'm glad you understand the time and financal dedication it takes to own a ferret, any animal really.
Pick up the book Ferrets for Dummies it will help you a lot previous to and during ferret ownership.