Now that I've just begun reading Greek texts in earnest, I'm trying to learn as much vocabulary as possible. I'm currently reading Attic texts (just finished Lysias' Murder of Eratosthenes) along side the Greek New Testament for variety. To aide in this I'm using Building Your New Testament Greek Vocabulary by Van Voorst, with which I plan to learn the bulk of its vocabulary.
I don't have any examples on hand, but it seems there are words used differently in Attic versus the New Testament. Is the difference subtle, or do I risk negatively impacting my ability to read Classical Greek? Also, does the lexical corpus vary significantly between the two? Are there popular Classical words that disappear later and vice versa?
Also, in terms of grammar, should I hold off on Koine until I've more deeply absorbed the syntax and morphology of Classical Greek? I'm aware that Koine is much more simple in syntax, and misses several grammatical features. I'd like to keep up with both, but I fear laming my Greek at such a tender age. For the record, if I had to choose between them, my choice would be Classical.
(PS Is there a Greek equivalent to Cicero in terms of canonical style? I've drunken heavily from the cup of Cicero in my Latin studies, and I think it has kept me on the straight and narrow in that respect.)