As a start, you will need more than one VPS. In fact, you are probably going to need several.

I suggest you investigate using nginx+php-fpm, and using a load balancer. If you use something like Amazon you will probably want to start more application servers when you expect all the load, and then stopping most of them afterwards. This'll save you some $$.

Use this: https://github.com/lucisgit/moodle-jmeter-script-generator to performance test your setup. It makes a jmeter script that will login and make forum posts. Tweak the numbers concerning ramp up time and delay between requests in the resulting jmeter script in such a way as to adequately reflect the amount of requests you think you are going to do.

When testing 700 simultanous connections, you are also going to need to run the jmeter itself on a reasonably well connected and quick machine.

You cannot be certain. Things that are resource intensive are anything to do with streaming (covered for you as you are using Youtube) and anything to do with the quiz. Assuming your users will login, select a page then watch a video hosted on youtube the performance overhead will be modest. However it is not possible to say exactly what the hardware requirements will be. For example if all 700 users logged in at exactly the same moment then a reasonable configuration might grind to a halt, but that is a very unlikely scenario. It would be more reasonable to assume that even if they all end up watching a video the login process would be staggered over a few minutes and the actual demand would be dramatically reduced.

Hi

Considering the statement "The only other activity that will take place [apart from YouTube videos] is comments or forum posts during the Q&A session", my guesstimate is that even a decent shared server might work, provided that it is not overbooked (plenty of reports in this forum) or does not have artificial caps ( https://moodle.org/mod/forum/discuss.php?d=215641 ).

But if you are serious about those "700 concurrent users" a shared server is a bad idea considering the risks involved against the added $ 32.- per month you need for a VPS. I would say a good quality VPS should be able to manage. Measure its network bandwidth and latency at various times of the day and week. Also see that it can deliver the full bandwidth from the (virtual) harddisk. A PHP accelerator is also a must.

That said, Moodle is based on the http://en.wikipedia.org/wiki/Moore%27s_law. You didn't mention the Moodle version you are planning, but if intend on joining the version run (a new major version every 6 months) you need to double the horse power every 18 months. Or buy ten times the power you need today, with that you can delay your EoL by 3 to 4 yeras.

Yes, this could be expensive. When you get advice on user numbers and Moodle it is frequently based on numbers over any 24 hour period rather than peak, so it is hard to calculate the cost and hardware required.

It's do-able. I would suggest an absolute minimum of a web server (Linux based) with at least 32G of RAM plus a second machine of at least the same size for the database. Run PostgreSQL if you can. Much more if Windows OS. Even better - two web servers like that, load balanced to be on the safe side.

Do note that this really is plucked out of the air and should be seen as a starting point. You would be much better doing a limited trial and monitoring carefully the load imposed.

Of course, to be blunt, your delivery model doesn't sound very sensible to me. Are you doing this in an attempt to stop people downloading the material? You do know it won't work, yes?