Like any creature a responsible breeder is always the best option for purchase in my opinion. However, a same gender pair of young healthy pet store rats can be fine if it's a decent pet store and doesn't keep ill rats, overcrowded in dirty cages.

Basically when you look at any young rat judge them much like a puppy or kitten. Clean, bright eyes, clean ears with no discharge, clean butt end with no signs of loose stool (check enclosure for this as well), fur shiney and laying nice to the body (no patchiness, no big fluffed guard hairs up, no scabs on the skin or obvious areas of their coat they've been chewing or picking at). Make sure the pet store has kept them strictly seperated by gender if you are buying a pair of females and don't want an "oopsie" litter of rats. They might be skittish but should never be mean natured or aggressively trying to bite you. Temperment is very genetic in rats so you want only nice tempered rats whether you are buying a pet or breeding them. Check the feet and tails. You want nice dry feet and tails....no damage and no sore on the rat's feet (bumblefoot).

No sneezing or excess red discharge from nose or ears. Eyeball the cagemates of the rats you want. Do they appear healthy as well? Are any of them sneezing/scabby looking, or obviously ill?

There are a number of quite nice big rat cages at PetCo. Remember your young rats will grow so you want a cage big enough for two adults (males get considerably bigger than females). You want a cage they can't chew out of or squeeze through the wire (when young).

Rats make great pets! They have almost no natural odor (as compared to mice, gerbils and hamsters). They do not bite as much as most pet rodents. They can basically eat most things humans can so will happily eat your kitchen scraps plus Becky's mix (pet store rat mixes are a waste of your money in my opinion). They are quiet and can be leash controlled while out of their enclosure (you can find appropriate leashes at most larger pet stores). As long as they have a rat buddy they are quite content with each other but will also love to interact with their owner. They live generally about 2 years or slightly more.