Well, at 200,000 users, that would make you a large Moodle indeed. Massive would be more like it.
For starters, remember the front cover of Hitchhikers Guide to the Galaxy, "Don't Panic!" Mainly because you have certainly jumped in, or been pushed, into a completely new universe.
1. Don't try to do everything at once. That will get you into trouble.
2. Stick with what you need to know before branching out for anything else. One thing at a time. And you are certainly being asked to run an Olympic marathon without any preparation.
3. "What I would like is to have all eLearning modules available to users as well as a record of their completed eLearning and classroom courses." Moodle cannot be all things to all people. What you are asking is not even remotely possible for that many users, unless you have a very small set of courses and everyone has to complete all courses. Records of achievement are recorded in the Moodle Database, and can be retrieved by students via the Certificate module. I would suggest all courses have Students be responsible for their own certificates, just too many Students for anyone else to do it.
4. "I would also like to have a general repository of documentation which would be available to all users. Is this feasible?" With that many students you are likely to create a lot of headaches for yourself if you try to store these materials on your own servers. Spread the load, put some on DropBox, Google Spaces, even One Drive.5.Are scorm compliant eLearning courses displayed within Moodle itself? Opened in a separate window? Either/Or, depends how you set your SCORM up.
6. Is tracking information displayed when courses are completed? Erm... tracking from that point forward or to that point? Add the Progress Bar to the Moodle and tracking information can be displayed to the student from the very beginning of each course to its completion, if you want. For an entire module completion tracking plugin, never used one so don't know. If you want to track a person's progress after completion of training, then perhaps Mahara may be a joint option.
7. What is the best way to understand the server size requirements? is this affected more by how many users or how much learning content is stored? Wow... erm.. perhaps the best way to describe it is...both. You will have somewhere in your system the name and ids for everyone you want to access the Moodle. That can be utilized as providing the required information to the Moodle. How this is done.. again, no idea, they won't let me near the servers to find out how, but there is enough information in Moodle Docs on how to set this up for a system admin person to work it out. But the User records for that many people are going to be gianormous, brobdignagian at the very least.
The second part of the question is a lot more difficult to answer. How large is the learning content? If it is a few GB then not a lot. If it is a few TB, then that is different. Then how much of the content requires response, how much response, is it just text, text and images, text images and video? Just text is likely to mean a 10MB allowance per enrollee. For 200,000 enrollees that is 2million MB, or 2TB, 100MB = 20TB. Suit yourself.
At this point, anything I say is likely to be nonsense, so I would suggest your IT guys get in touch with the Open University IT guys and see if they will help. Tim Hunt and Sam Marshall are good guys, and might be willing to point you in the right direction - may not have time to directly help but if they do speak, listen carefully. BTW, the OU has something like 300,000 users, last I heard.
Good luck and welcome to Moodle.
PS: Howard is also one to listen to carefully and a Moodle Partner will take a lot of the sting out of trying to take on something you know nothing to little about.