Depends on your level of expertise, are you a competent programmer and know or willing to learn PHP? onyl way you can get a truly satisfactory result is to write everything yourslef, somthing not for the faint hearted!
I created a similar site a while ago for a small organization, where different users had different accecss levels and restricted to certain pages accordingly. quite easy to create a login panel and password check,every person will have a access level code, simply 1 2 3 4 5 will do! and load the corresponding menu, but hell i might as well be talking a different language to most people..
One site you may find useful for this type of thing is click here look at thier own tutorials on membership systems and others very very useful.
If you cant programme much and so on and dont want to go down this road, then im afraid i am a bit lost as to what you can do. well for tonight anyway as you can see by my typing and spelling i should have been in bed ages ago!