Rather than an LMS (learning management system), which may have more features or be more complex than you need, you can also consider a membership plugin. This will serve to lock the content from non-members and allow you to setup different levels of access. The plugin will also allow you to drip content instead of providing access to everything at once (for example, they access X on Day 1, Y on Day 3, Z on Day 5, etc).

But if you want to quiz and test your clients, then you should definitely go for an LMS instead.