The quick answer is everything you're asking for is possible and fairly easy to implement. You're scenario and desire to keep things scalable on a large network is the main purpose for WPMS. We've built over a dozen client networks some with more than 700 installations on one network. We have built robust tools like the neversettle.it/ns-cloner-pro which allow you to clone, copy, update, migrate, etc. with one click inside your network.

With MS you can run an admin level codebase for all sites while maintaining unique content as well as unique functionality and design if you choose. With domain mapping you can also have a custom domain per sub-site if desired as well. I.e. instead of having dealerA.yourdomain.com and dealerB.yourdomain.com you can have dealerA.com and johnscardealerB.com

Once you have correctly configured your network and have tools like our Cloner the possibilities are endless.

Cheers,