You're getting the hang of it

You can spend as much as you like, and it will make life easier, but the price sky-rockets quickly. The trick is to make up the ground with fieldcraft and there are guys on here that get amazing results from much shorter lenses - but you have to sit in a hide for a week with a bag of peanuts dangling from the end of your lens.
TBH, unless you are very confident of where you're going with this and bearing in mind that few of us can afford to make too many mistakes, I would go for the 70-300 VR and see how you get on. It's also a very good and versatile lens for all sorts of other things. It will sell on very easily for good money (use the classified section here, it's excellent) and if you start by buying a good used lens, then you could upgrade later without losing a penny.