I think seems this is your first you should look at getting the Maxi rider. It will last your child much longer then most. Both my boys were too big for there seats by 2 & a bit. Even though the seat is recomended for up to 4yrs (18kg) they were just to tall for them by 2. I now have my 4 yr old in the Hipod & my 2 yr old in a booster seat with harness.
If I could go back to the beginning & buy again I would get the Maxi rider as it would still be ok for Evan & Glenn & I wouldn't have had to buy extre seats that don't have the 5 point harness. But I wouldn't buy the maxi rider now as its a lot of money when I only get 1 stage out of it instead of all 3.