Chinchillas are a lot of fun. And, I'm really glad you're doing the research before hand.
Let me start by saying, though, chins are not cheap pets. They don't cost a fortune, but they do need their supplies, as I'll detail below.
Though by no means am I an expert, I have had 6 chins over all. Some I "inherited" through my boyfriend, and one was born to us by a very sneaky rendevous of two of ours. So I've had a pretty wide range of experience!
Chinchillas are like most other pets. As you said, if taken care of properly, they seldom need to see a vet. However, like all pets, they can have other things than sickness. For example, my first (and favorite, shh!!), Sarah, was never healthy. She was small and epileptic. She died young, at only about 8 years old, and had to see the vet quite a few times near the end. The good news is, all our other chins have never really had to see the vet! We took them down just to check them out, but they didn't have to.
To answer your questions:
Yes, chinchillas make great pets, if that's what you like. Cage animals are of course different than a cat, but as an animal family, I'm sure you know that! They are pretty easy to take care of. They need attention, so they're not one you can put in a cage and forget. They need food always, they are grazers, so their dish being empty isn't good. It's not bad, they can go awhile without food, but they shouldn't go too long. They love their treats! They need regular dust baths (the coolest thing ever to watch!!), and they must have plenty of things to chew on. Without a dust bath, their coats get greasy, and that can make them sick. Without things to chew on, their teeth will keep growing and can grow back in to their mouth and make it so they can't eat, and can even grow in to their brains. But trust me, you give them stuff to chew on, and you have nothing to worry about!
So, the supplies you will need. A large cage- they need room to run and jump. A wheel (solid, no spokes to get caught in!) or a saucer for them to run in is GREAT and loved by all chins. Bedding, NOT CEDAR. Places for them to hide- you can buy these from a pet store, or make some. Mine have heavy duty PVC pipes (among other things!) to play in! Cardboard is great, because they can hide in it and chew it! Though cardboard is not thick enough to wear down their teeth, and they do need wood to chew on. They do best with Timothy Hay supplements to their diet. Raisins are a great treat for them, but you only need max a few a day, and one every few days is good, too, so they won't cost too much. You'll also need dust bath for them, and of course food and water dishes- I recommend glass water bottle and heavy duty PVC food dish that attaches to the cage. Those are both investments you'll never regret!!
So, the costs are pretty much all of the above. Most are costs are recurring, because chins will chew and destroy everything over time. Dust bath, food, and chew toys need to be bought most frequently, and bedding depending on if you buy small bags or huge ones. Followed by cage acessory replacements, like ledges and huts.
Chins LOVE their freedom! I have a picture on here somewhere of "chin kingdom" that we had set up for ours, so they had freedom almost every day for a few hours. Let's see if I can find it...
So they just had free run of that entire room, and each others' cages. Yes, they do poop randomly on floor. Always fun.
But, most "potty train" themselves when it comes to pee, and they tend to only pee in the back bottom corners of their cages, which is nice.
Of course, I can't say how Bella will be with the chins, though my Evie is pretty good at ignoring them. Most chins will basically ignore a cat if the chin is in their cage and the cat is not close to them. However, during freedom, each chin is different. Sarah was fine with all my big pets- she'd walk on their heads, in fact! Fed pretty much ignored them. Toku either ignores them or attacks them, depending on her mood. Nigel would always attack!
Whew, ok, I think that answers your questions. I know there are other people on here who know lots, too, so hopefully you'll hear from more than just me!
But, if you should ever have questions, I'd be happy to help you out, so feel free to PM me anytime!