I don't think that at the start there's such a big difference as the verb and noun conjugations/inflections are all the same. The plainest Modern Hebrew would still work for Biblical Hebrew, and besides, it might be easier and quicker to get to a pre-intermediate level at Modern Hebrew then switch to Biblical, since Modern Hebrew is closer to modern European languages, which will allow you to adjust more comfortably and with less difficulty. In general, yes, there's a big difference in the subtleties of grammar between Modern and Biblical Hebrew, but on the whole, starting with Modern Hebrew won't set you back at all (and it would be easier to practice).