Am answering this on phone do can't look back at your post so hope I cover everything.
Gerbils need to live indoors so a shed etc too cold for them. They need somewhere warn, but away from extreme heat or cold. A temperature that can be regulated to a degree.
They are better off in pairs or small colonies and thrive with the company of others. This being said you need a big enough cage to give them space. As with humans there are some people we just don't get on with and sometimes gerbils will fight others. If this is the case they may need to be separated. V small chance and not reason enough not to get two or more.
They are prolific chewers and burrowers so don't get a rotastak cage (the tube ones) or really the metal ones either, go for a gerbilarium or a clean fish tank that you can put loads of sawdust in so they can burrow and make tunnels but not chew their way out of.
Three main reasons they (like hamsters) will bite: if you pick them up straight after waking, if they can smell food on your hands or if you grab them from above rather than scoop them up. They can also shed their tails so should not be picked up by them (defence mechanism for the wild).
They are awake a lot more during the day than hamsters and tend to be more inquisitive, however they are quite fast and therefore are not suited to very young children.
In regards to vet bills: they don't tend to have and ongoing issues however there is always the chance of tumours, dental problems, abscesses, and other issues so its best to be prepared for the slight chance these could happen.
They live for anything up to four years - average two to three and I think they're great pets. The tunnels they make and their personalities are great! They do require handling and cleaning at least once a week. They can cause quite a bit of mess so bear that in mind as well.
Gerbils are relatively cheap to keep, after the initial set up costs. I agree with sparkle, the only con I have found is that they are fast (the two we have had have been). We have never been able to just pick out of the cage for a cuddle, they would have jumped and injured themselves. Ours goes in a play pen, we sit on the floor then pick up, if she falls, she doesn't fall very far. But, they are so cute and funny, love treats and are very easy to spoil. Our current girl loves the occasional gerbil chocolate and bit of plain biscuit. I have only been bitten when offering a treat, they thought my finger was the treat too.