Organizational Dilemma. Buddypress, Communities, Multisite, combo of all?

I'm setting up a classroom site. Some classes are free, some are paid. I thought I would do it all with Buddypress, then I discovered that BP groups actually don't have anything in them (unless you add a forum) no pages or blogs are actually housed IN the group. So protecting the group seems like a waste, because the groups just give people an identity, but they don't give them access to anything.

But I don't want all my users to have a blog, they just need an identity so they can interact.

I installed multisites so I could use them to create sub areas to house the classes. My USERS will not use multisites.

Someone with experience please let me know if this is correct, because I'm spending an awful lot of time trying to like the features here but they so far aren't as good as Ning.

1. Set up main site, install Buddypress
2. Set up free class site:
3. Set up paid class site:
4. protect paid class with membership plugin and get people to pay for access to everything on the site (does this work? if so, how?)
5. set up forum on paid area for those students to ask questions

I also want to use Classifieds so some of my users can sell class materials, so do those go on the main site?

Is there a reason for me to use communities, because they seem to have more/better features than Buddypress?

You can see I'm kinda confused.