"Nosotros compramos un carro" translates "We buy a car" AND as "We are buying a car." Both.
In English we have two ways to say the same thing for whatever reason. Spanish lumps then into one way.
An "estamos" is in no way needed. It is unnecessary. Including it is like saying in English, "We are are buying a car. Double talk.
Besides, when including a verb after a verb an infinative would be used. I.e., Comprar.