I'd like to say a bit more about Bart Ehrman's oft-repeated contention that the traditional authorship of the Gospels is wrong because they were written in literary Greek, which would be impossible for Aramaic-speaking peasants to emulate. Ehrman also uses the example of Josephus, who learned Greek later in later, to write books for his Roman patrons. Yet Josephus also admitted that he needed assistance.
One of Ehrman's many problems is that he doesn't stop to consider obvious counterexamples to his claims. He lacks a flexible mind.
Consider the immigrant experience in America. Take the stereotypical case of adult foreign speakers (parents, grandparents) who move to America. Sometimes they bring little kids with them. Sometimes their kids are born here. Or both.
Adult immigrants often struggle with the language of the host country. They may speak broken English. That's in part because many of them are too busying working (which is admirable) to have time to study the language. But the primary reason is that it's very hard to master a new language in adulthood.
By contrast, young kids sponge up languages. If their kids are born here, or come here at an early age, they can learn English just by listening to TV shows and hanging out with Anglo playmates.
Kids of immigrants typically speak fluent, idiomatic, unaccented English. Their command of conversational English is flawless.
Now, that's not the same thing as literary English. However, I daresay that if you have a native command of the spoken tongue, it's much easier to learn literary or academic English. You have that foundation to build on.
Ironically, the Jewish uppercuts education that Josephus received was an impediment to his learning literary Greek. From what I've read, Jews of his station didn't consider Greek to be a prestige language. After all, they had Greek-speaking slaveboys.
By contrast, a Jew who learned street Greek growing up would actually be in a much better position to learn literary Greek later in life. Keep in mind that for many Diaspora Jews, Greek was their mother tongue. And some of them moved to Palestine. Take Barnabas, a native of Cyprus, who was the uncle of John Mark.
Perhaps Matthew was a tax collector because he was bilingual (or polyglot). Surely that would be a marketable skill for a tax collector in Palestine.
Finally, although Matthew's Greek is a notch above Mark's Greek, it's not fancy Greek.