Need consultation on Magento by an expert

**We are only seeking consultation/advise - no software programming, no coding, no customization. We only need your expert advise and to help answer some questions we have.** Here is what we want to do We want to customize the open source Magento shopping cart system. We want to build it so that we can have users create new installations of Magento on the fly (i.e. with a few clicks). The new Magento installations will contain our modified/tweaked Magento code. We want answers from you on potential problems we may have and others. ## Deliverables Here are our questions: -Overall do you see any issues with our plan? -Multiple sites with one admin (how to split the admin to multiple clients) -How can we upgrade our core magento system without breaking any features we've added; and vice versa -Current admin interface is too complex. Do we build an admin overlay or edit magento's current admin interface? -How to build on Magento with most flexibility -Whats the best way to have multiple developers working on different modules -For spawning new stores (store sign up), what web host requirements will we need (i.e. Shell Root Access, security, permissions,etc). -Maintenance (routine backups, remote backups, restoring backups of specific clients any point in time).