Although I would wholeheartedly endorse a recommendation to learn classical
Greek first, the fact of the matter is that I started with koine and classical
came later, after graduation from seminary. For those who might contemplate
the same thing, I can say that if you are willing to do considerable catch-up
work (I ended up with a second bachelor's in classics, something not everyone
would wish or find necessary) learning classical is not a problem. I sometimes
found myself taking mental notes of an odd grammatical term that came up in
class and then looking up and studying the term later at home or the library.
One should normally ask the instructor a question under such circumstances,
but I was often too afraid to. Even so, I did very well and went on to grad
work in classics.