1) No, everyone does not need it, but it makes it a lot easier on the commish if they do. If they don't, then they have to tell you everything they want done which you then have to do and you just know you're going to have people whose instructions are not clear or they didn't think of something which you'll then have to deal with. It certainly can be done, but it really is easier if they do it themselves.
2) If you get someone's updated rosters or you create the players yourself, sure. I doubt you'll get more than the top notch prospects though. Most modern leagues realize that if they're going to last longer than a season that they'll be using fictional players soon enough so it's not that big of a deal.
3) Well, part of the websites you're probably seeing are the reports OOTP generates. Part are probably forum software that you can install yourself (some are free like phpbb3, some like vbulletin we use here cost money). Then the rest are pages you create yourself. You can also go with a webhost like fidel's where he provides you a template that will work for most leagues and you can customize it yourself. So it's really up to you. You can use whatever webhost you want (if you already have one for something else then it'll probably also work for OOTP) or you can use one where they have much of it already set up for an online league. I started
a thread awhile back on webhosts here.
EDIT: They can be as easy or as difficult to create and maintain as you want. I help administer the CBL and I cop out and just use a portal with the phpbb3 forum software. I'm pretty familiar with it though so it's fairly easy for me to customize it how I want and I don't have to bother with putting a lot of time into coming up with something that looks totally unique. If you do have the time, interest, and ability in coming up with something unique (like the guys at FOBL) then that's probably the best way to go as it can really look sharp and attract a lot of interest. If you don't have all that, then you can certainly go with a template and still have a great league, you just might have to find other ways of standing above the crowd. A modern league generally has little difficulty in finding people though as most people seem to be modern league fans rather than fictional fans. They may or may not suffer from more turnover after the first season though since they end up looking less and less like real life as time goes on.