How to create a WPMU Community with customized dashboards and editing pages

I need some strategic advice and I'm sure your wonderful team could bring a lot of help. :wink:

Here's the deal, I'm building a community (blog network) for a non profit organization here in Switzerland, using WPMU + BuddyPress...

The problem is that our users are really NOT tech savvy at all, so my goal is to provide them with process as seamless, clear and simple as possible.

There is a few important things I need in particular :

1. I need to be able to customize the admin dashboard (to remove unuseful stuff for them to avoid confusion).

2. I need to be able to customize the admin top bar. (same reason) Especially, I don't want them to have an access to the main website.

3. I need to be able to customize the edit page (when editing a page or post) to remove all the crap on the bottom and sidebar and just leave the most important functions.

4. Most importantly, I'd like that when they create a new website, the website they create is already full with dummy content, custom settings, etc... Is there a way to create a "demo" site so that they don't have just an empty WP site when they get started ?

Hope you can help, thanks a lot in advance, and do not hesitate if you have any questions.

Any strategic advice on the best way to proceed and plugins/themes recommandation would be great.

Best regards,

P.S : My website is at the start, but if it's of any help :