Online and face-to-face courses - booking and invoicing - how to simplify?

One of the clients has asked for a website, and one of the features offered by the site will be the ability to **book in and pay for the courses**. The courses can be either **online** or **face-to-face**. For the online-courses, it has been decided that Moodle will be used. The courses will be **paid for via a CRM**. The same CRM will be used for other purposes as well as invoicing... We were hoping to enable the **booking process and browsing through the courses on the Drupal website**, but will this become too complicated? Drupal -> CRM -> Moodle? Also, because there are a certain number of spots for each course, the Enrolment form in Drupal needs to be able to display the number of available spots for the selected course. Any suggestions on how this can be achieved?