Which modules should be used?

I am trying to build a website that has following features:

• An organization buys access to a specific service, e.g. certain quizzes, ability to schedule their staff, etc.
• The organization can add employees to their organization
• Managers/Organization can see different nodes than employees, and will have different permissions to different parts of the system (e.g. manager can add employee, employee can’t add anybody)
• Admin is able to limit how many employees can be added by the organization (should be predefined by the package they buy).
• The ability to add new features to the existing services (e.g. the organization bought first quizzes, then wants to add calendar function, more quizzes, more employees etc.) without having to setup new organizations.
• Ability to lock the IP address of organization and employees. I want to ensure that the information can only be accessed from the initial setup IP address.
• Would it be possible to lock the IP address of the organization and the employees separately, or allow a 2nd IP address for employees?

What modules would be recommended?

I thought of using Ubercart as shopping cart, the quiz module for the quizzes, either taxonomy access or TAC lite for restricting access to the quizzes. I would probably need to setup an organization role and an employee role to predefine who can do what.

I’m not sure about the organization part though. Is OG the best way to go, or are there other possibilities?
Also how can I ensure that the organization can only add their employees to their organization and how can I limit the amount of employees?
What is the best way to make new services (packages) available? Especially how would that work if the new package would include a role or permission changes?
How can I set it up, so the organization will automatically get the access to their package and the employees what belongs to them?

Thanks for the help.