I've been keeping rats for 10 years now! So you can see that I love them very much!
As small pets, they are full of character and will readily bond with an owner. They're mischievous and nosey and never cease to amaze me. Being a bit bigger than many small animals, they're easy to handle and cuddle.
Girls are *generally* more bouncy and nosey, and boys are often rather more laidback and cuddly. I've had both, and I personally prefer the does, who are very active, but I have one neutered buck, Mocha, who is very, very lovable. I've had bucks who were hyperactive though at the same time, so you can't always stereotype. The bucks do get bigger than does though.
Rats do suffer from some health problems - most notably respiratory issues and tumours. Because of this, you're really best to get them from a reputable breeder than from a pet shop. Well-bred rats are also more likely to be friendly too, as they will probably have been well-socialised. If this doesn't worry you too much, there are often lots of rats in rescue centres - 6 out of my 8 rats are rehomed from elsewhere.
Tanks don't make good housing for rats, because they don't have good ventilation, and as rats suffer easily from respiratory problems, good air circulation is really important. My rats (8 of them) live in a Ferplast Duetto - an indoor aviary. They lived in a 100cm x 100cm size cage before that. Obviously, for 2 rats you don't have to get such a big cage, but the bigger the cage the better!! I have some pics of my cages if you want to see examples.
Rats are very sociable, so yes, you need at least 2 of them. 3 is a good number, as if one dies, the other is not automatically left alone.
Food-wise - they need a good quality mix (I've posted about what I give them in a post about home-made mixes a bit further down)
along with fresh food also. Mine have carrots, peas, rice, pasta, curly kale, apple and lots more!!
If you have any more questions, please feel free to ask.