
I do love rats, and have had them on and off for almost my whole life, starting with my first when I was about 14.... which is humpty-hum years ago. I first joined the National Fancy Rat Society when it was only 5 years old.
The boys are bigger, more laid back, and cuddly, although I've always loved the girls best as they are sleek, lively and by far the most intelligent! and naughty.... in a nice and funny way.
There are a lot of rescues with baby rats available and providing they have been handled A LOT from very young, then they will be fine for a beginner. I would be cautious about taking on adult rescues unless they are bombproof. Like puppies, the more socialisation you put into baby rats, the better they will be when old enough to go to new homes. A lot of pet shop rats, and those from casual or inexperienced breeders, won't be, and there is a ton of difference between well socialised babies and those who are nervy at the start (as you'd expect). Mine were handled from birth - a good natured mother from good tempered lines will not mind at all... just as you'd expect from a good dog breeder, with a good bond with their bitches.
Rats do have a number of inherent health issues, the most usual is a pervasive respiratory issue, Mycoplasma. Good breeders will aim to strengthen immune systems and only breed from those with good resistance to disease. This can't be said for rescues or pet shop rats.
The National Fancy Rat Society can give you more information, and holds a list of breeders; though they can't vouch for everyone on their list, it's a good way to find breeders near you if you choose not to rescue.