Currently I have around 3000 users and 10 courses and am experiencing issues with premier shared hosting that has no official restrictions on concurrent users or no. of emails etc. The shared hosting companies mentioned by you have restrictions. I am in the process of migrating to a Moodle partner.
In your circumstances, I think a dedicated server should provide you great value for money.
The situation you are describing is definitely what you would call "mission critical," so you need to consider to whom you would turn if you have a problem. You have too much at stake to put on a shared server.
I use Moodle for ONE class with 28 concurrent users (just slightly "mission critical" compared to your situation), and I get nervous every time they all get online at the same time. Once there seemed to be a slight problem with the server keeping up in the forum entries, but quizzes have always worked well. The point is that on a shared server, there is never any real COMFORT because your site may fail, of course, when it would be most problematic.
As it is, then, it's necessary not only to perform the automatic backups every night which is no problem for just one class, but my "comfort level" requires that I download the backup somewhat frequently, at least two or three times a week, and always after major assignments.
It's definitely too expensive for me at this time, especially with just one class, but with the mission critical situation you just described, go with a Moodle partner!
Finally, there's always dedicated hosting but at that point, you've probably passed the cost of using a Moodle partner and enormously increased the amount of administrative work that you (or your webmaster) would need to do.
BlueHost recommends VirtualRoot.com but I can't vouch for them personally (I work for BlueHost but have no direct affiliation with VirtualRoot). From looking at their website, you would do fine with the smallest package (fairly comparable to BlueHost's package) but from their FAQ it seems like the bigger the package you pay for, the fewer other users will share the machine with you.
I just went to check the prices on some of the Moodle Partners and at first those prices seemed outrageously ridiculous to me but then I realized that its a lot less than paying someone a full time salary to manage you're Moodle server. So if you know what you're doing, don't use a Moodle partner. If you don't, then you might consider it. In the meantime, any reasonable hosting company (shared or otherwise) will give you a 30 day money back guarantee. So go check out a few. See how easy it is to install Moodle. See how easily you can get tech support. See if they'll help out any with Moodle (the answer will generally be "no, its a 3rd party program, we can't help" but you might get lucky). Maybe what we really need is for someone to narrow down all the non-essential items in Moodle that have a tendency to bog down the server and then come up with a stream-lined install (ie with fewer resources/accessories but faster access times). Or at least a guide to stream-lining a current install. Hmmmm....
You will need a dedicated server, definitely not shared hosting. Purchasing your own server will be the best value for money if you have staff to install LAMP+M. If not, get a dedicated server plan from a Moodle Partner. Note also that we have found internal LAN speeds with an on-campus server to be 2x or 3x faster than externally accessed off-campus via internet.
Our university has 5000 students and will eventually have about 200-300 courses per year. We run on a Dell SC1420 server which from our experience will handle 500 simultaneous users at a 50% load. Specs as follows...
- Dual Xeon 3.0ghz processor x 2
- No OS
- 2 GB memory
- 80 GB hard drive x 2