By all means, find out how you will be billed ... daily usage ... caps ... burstable usage means higher bill, etc.. But you have no data/info yet, so guess!
Between Amazon and Google (dunno about Azure) think I'd go with Amazon. Google offers similar services but they base their billing on usage ... and you have no baseline data ... yet.
Correct if wrong (anyone), but thought Amazon offer caps (customer decides those limits - memory,space, etc.) and thus one would know what the monthly lease would be.
Think about what you are asking ...
"For instance, if I rent 8GB RAM with processor of 4 cores and 500GB of HD, would be it enough to guarantee that 5000 users can use Moodle (worst case) in a time and maintain my server OK. I know that 5000 users in a time would be difficult to happen, for this reason i was thinking in a cloud provider"
8Gig RAM, 2-4 cores ... that's OK to start ... 500GB HD ...
I'd opt for 1TB of an attached drive/device ... better to have more than enough than not enough ...
The size of those will grow with usage ... and since you don't know how and what teachers and students will be doing ... again ... better to have more than enough than not enough.
Maybe, the provider could guarantee the stability of the server."
That's a question to ask provider. Of course, if you are talking to a sales person, they would more than likely tell you anything they think you want to hear ... to make the sale.
The system you configure to rent will be managed by you ... backups therefore, important. Stability is your responsibility ... IF ... the one server houses not only the code and the data directory, but the database. DB server will need to be tweaked from time to time ... at first, maybe fairly often ... as usage increases. Predict you will be increasing the amount of memory the DB server uses ... the number of connects that are allowed, etc. 1st month it's up .. whatever you've chosen ... good idea to monitor daily until you learn how moodle is being used. You decide how often you monitor ... think I'd try to head off complaints, email messages, troubleticket kinds of things as much as I could.
No providers (cept Moodle Partners) will go application deep in what you lease. That's your job.
I would check into what services are provided for images of your entire server, however. That's for major disaster purposes - you should be using that often ... call it 'extra insurance'.
Now if the above isn't clear enough, step back .. research usage more ... then rethink/assess.
Above is free advice offered in the 'spirit of sharing' ...