a new one
Este auto usado es tan bueno como uno nuevo.
This used car is as good as a new one.
Puedes elegir uno nuevo, pero que sea bueno.
You can choose a new one, but make sure it's a good one.
He perdido mi reloj, así que tengo que comprarme uno nuevo.
I've lost my watch, so I have to buy a new one.
Será mejor que compres uno nuevo que intentar arreglarlo.
You'd be better off buying a new one than trying to fix it.