I think as long as it meets Australian standards its fine. We have a capsule as well as a Mother's Choice car seat which is from birth to 4 years. After the 4 years we'll just get him a booster seat.. i know my nephew was embarrassed and was teased a little when he started school at the age of 5 to be in a booster, i think keeping him in a "baby" car seat would be worse.
They say to be careful when purchasing second hand car seats, unless you know the person, as to whether theyve been in an accident or tampered with.. the car seat is something i wanted to definitely buy new for that reason.