Wordpress Multi Site advice

Hmm I can suggest you have to make theme for it! and each category or taxonomy have its own style as you mention above!

Hmm thats a solution. I planned on using some Elegang Theme. But how ressource intense is 10 wordpress installations rly?

The site wont recieve more than 100 visits a day max.

Thanks for suggestion

Multisite will be overkill if you just need an inner page "with different styles". WPMU (WordPress Multisite) allows you to create serveral sites sharing one WordPress core, so you will have let's say, mydomain.com/site1, mydomain.com/site2, mydomain.com/site3...and so on. Each siteN (N = 1, 2, 3...) is accessible from the main network dashboard. So we are actually talking about individual sites but with one WordPress core.
Unless you are planning to have different sites, with different content but under the same network...WPMU is not the right choice. If you need design flexibility, a theme may require some tweaking, I recommend you to look for theme frameworks like Genesis, Headway, Catalyst...etc.

Well WPMU might work

I just want some navigation method to switch between the branches, and the main menu would then switch between the chosen branches and show the content for that specific branch.

So in theory its gonna be 10 sites, but in 1 design. Think WPMU can do this?

Yes, your sites can share the same design.