I think you can try with the shared package at first and if you will grow then tou can consider in upgrade to VPS or even dedicated server. All of this depends on engine your want to run, how much resources your portal will be using. I know some cases when account on shared server had more then 250k if unique visitors per month and this person wasn't overloading the server
(Bandwidth he was using was almost 2TB). Also I know the client that had less then 5k of unique visitors per month (Bandwidth he was using was less then 100Gb) and Good VPS was not enough for him.
So I think you should check the shared package first and only then think about upgrade to some VPS.