I understand that Jesus and his followers spoke ancient Aramaic.
Yes. But it used to be assumed that the region was a 'backwater' of the Roman empire, until more and more archaeological evidence turned up pointing to the contrary.
It now appears that Greek was a common second language to the upper echelons of Jewish society in Judea. If you lived in a town or city, or were involved in commerce, you spoke Greek. It's quite likely that Greek was a second tongue to Jesus and His companions.
However, our earliest biblical manuscripts are in ancient Greek (ancient Hebrew for some of the OT).
Yes. It's unlikely they were written in Aramaic and then translated.
Any Aramaic translation would have occurred after the original manuscripts (they are not Jesus' original words, but a translation of the most original words we have into Jesus' language), and thus, may or may not be as authentic as the Greek/Hebrew. Thoughts?
Sounds reasonable.
Luke we know is addressing a non-Jewish audience, and the literary structure of his Gospel reflects Greek style. The journey motif Luke employs (half the gospel is on the road to Jerusalem) is a common motif in Greek literature.
Whilst Luke thinks in Greek, Matthew thinks in Hebrew. His Gospel is a really sophisticated piece from a very well-educated Jew. The Gospel follows a narrative structure that is a favoured Hebrew technique. Why write in Greek then? Because that was the language of the educated Jew. We have sacred texts transcribed into Greek because Hebrew was something only the priesthood read, we have funerary inscriptions in Greek. Hebrew was becoming the language of the synagogue, but not the street. Aramaic was a dialect.
There was a 'sayings gospel' of Matthew, according to Papias, 'in the language of the Hebrews', which might have been in Hebrew, or Aramaic. I think Matthew combined this logia document with Mark.
Mark is not so happy in Greek. He uses Aramaic terms. It's notable that both Matthew and Luke incorporate Mark's grammatical errors, which suggests a reverence for the text. If we accept the most likely source for Mark as the oral tradition of Peter, you can see why.
John, again, is educated in Greek, but it's clear John moves easily in the higher social circles. He's known to the Jewish authorities, he's allowed in when Jesus was being questioned after His arrest, while Peter stayed outside.
Personally I think Jesus spoke Aramaic to his friends and Greek when dealing with Gentiles.