I'm not attacking you, just informing you, but this site is a rat rescue forum. Members here will always push for rescues before breeders, but we'll understand if you have to get babies due to no known rescues near you or said rescues have no rats available.
I always get rats in trios, but that's just my preference.
Also, female and male rats do behave slightly differently. Female rats are a lot more likely to explore, go on an adventure and always want to play. Male rats are more likely to cuddle up with you after a little bit. My heart rat would fall asleep on my chest, content to rest and be adorable. Needless to say, there are always exceptions to this "rule." You could have an active, playful male or a relaxed, cuddly female.
If you go to a breeder, let the rat choose you. Funny enough, when I got my newest rats, the littlest one chose me. I was concerned my adults would accidentally hurt him due to his size, but he was licking my fingers, grooming on my hand, cuddling in my sweatshirt. He had chosen me
(Long story short, I waited a little bit longer than normal to introduce him, just to make sure he'd be big enough to meet them)
However, earning the trust of a rescue rat is perhaps one of the most humbling experiences. Watching them because more extroverted and excited to greet you is an absolute wonder.
Here is a rat cage calculator that'll help you figure out if the cage you're looking for is an appropriate size.
https://www.ratcentral.com/rat-cage-calculator/
Also, you can have a mixed gender group of rats, but only if they're spayed or neutered. I've seen a bunch of females have a neutered male rat as a companion. Everyone on here, including myself, would recommend spaying female rats for health reasons.