Hello Paulo. I am a native greek speaker and I can assure you that it's not necessary to learn ancient greek in order to be able to speak modern Greek! yes, ancient and modern Greek have lots in common and when a native modern Greek speaker tries to understand a passage in ancient koine and classical greek, one understand many things, though not all of them, because there are differences in vocabulary, grammar and a huge difference in syntax!!! Greek, either modern or ancient, is a beautiful language. Choosing between ancient and modern Greek depends on yours interests. if you want to read and study ancient greek classical literature like Homer, Xenophon etc I suggest that you take ancient Greek lessons. but if you want to be able to speak and write in greek, learn modern Greek! by the way modern Greek is much easier compared to ancient!  I hope I helped you [emoji][emoji][emoji] καλή σου μέρα! [emoji][emoji][emoji]

A Greek speaker should answer this, which I am not.

Being a native Modern Greek speaker makes it easier to learn Classical and Koine Greek.  I don't think the difference between Ancient and Modern Greek is as extreme as the difference between Old English (Anglo-saxon) and Modern English, for example.

Modern Greek speakers still need to study to learn to read Classical Greek.

If you are interested in either Modern or Classical Greek, choose one of them and study it on its own.  Trying to learn both at the same time would be crazy. 

Also, students who think they "know" Koine Greek because they can read the New Testament in a mediocre manner are totally lost when they try to read Homer, for example.  

 But, like I said,  someone else could give you a better answer.

 K P,

I mean that trying to learn Ancient Greek and Modern Greek at the same time, starting from no knowledge of either, would be a bad idea.   I would get a solid foundation in one before starting to learn the other.