If a rat comes up and licks your hand you have found a very sweet rat. Check out the rat's coat. Make sure there is no discharge coming from the nose, mouth or eyes. This is an unhealthy rat. Go to another pet store.
Male rats are larger than the female rat.
The rat's lifespan is from 2-3 years. An aquarium with a secure screened top works great. Metal cages are ok, except that they can corrode and the bedding gets all over. Rats' little legs can become trapped in wire cages where the gauge is too small. If the gauge is too big they can escape.