"We buy a car" = "Nosotros compramos un carro"
"We are buying a car" = "Nosotros estamos comprando un carro"
"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.
It can also be “We bought a car”. OK, so this is an early lesson and you may not yet have seen the past tense but it is worth knowing about 2nd person --ar verbs often being the same for present and for past..