Advice for this Multisite Set-up Please

I am planning a new multisite network and would appreciate any thoughts about which plugins I should use to achieve my objectives.

The main site should have a search form that will search all the contents of the sub-sites.
Preferably with a custom search form - using drop down options if possible.

All entries in all sub-sites need to use custom CMS or other way of ensuring they all use the same fields.
I'd also like the admins to be able to easily add photos to the entries without knowing how to use wordpress.

I need a stripped down dashboard - bare minimum.

The sub-sites will each have similar search forms but will only search that site, not the entire network.

I apologise for not being best at explaining what I want, si I've attached a brief mindmap.

Any ideas would be appreciated.

Martin