It comes with a base, so is completely functional out of the box, but I recommend purchasing an additional base for any other car your baby will be riding in, it just makes things so much simpler. Hope this helps, it's a great seat!
Base comes with the seat, no need to buy any additional items seperately. A nice once, better than evanflow. Specially this does not damage your car interior, no need to keep any padding or towels under the base.
is not recommended with out the base but it can still be used with safety belts over the front part of the car seat, just be aware this model already expired by the manufacturer. get a new one, it comes with the base included.