my site will basically be a phpbb discussion forum where at maximum there will 100 users at once online
Bandwidth is irrelavent here. Buy a package from a host that has packages built for application hosting. What you are really buying is processing power, bandwidth numbers are irrelavent really.
Maybe look at a provider like medialayer??
If you have 100 users online simultaneously now and expect to grow to 1500+, your best bet would be to buy from a shared provider that can actually accomodate 100+ simultaneous users today on phpBB (very few can). Then that provider should be able to seamlessly upgrade you to a a more dedicated solution as your requirements countinue to grow. Based on your comments, you will need fully managed solutions throughout the process, so ensure you have some sort of revenue generation associated with your site early on, as it is not cheap to host large forums.
having said this, you could also take the route of growing your forum without generating revenue, then buy one of the massive packages for $10 and wonder why your forum is always down or working really slowly
Always tough when you are just starting out, so make sure you research who ever you are going to go with and make the correct decision for your specific requirements (ie DB driven forum site with expected growth)
Best of luck and hopefully this information turns out to be helpful to you.