Putting it all together
As you work through the process of planning your site, you'll find that all these aspects influence each other. Change one, and you'll have to change others. That's OK; when you're on your fourth or fifth iteration of the plan, you'll understand why the actual development of your site will proceed a lot more smoothly if all the questions are answered first.
And you'll end up with an elegant, efficient site that starts to pay for itself sooner, not later.