Moodle forum posts in English language are best here.
If "Solo hablas español", tal vez te convenga más el foro de Moodle en español .
The best hosting I would recommend is a Moodle partner in your own language and, if possible, in your own country.
300 simultaneous quizzes might be very hardware-intensive (expensive), but less so if the questions are shown one at a time and the quizzes are slightly spread in a few minutes period.
Will the students use a gigantic computer lab?
Will they use their own home computers?
Do you have an example quizz to illustrate how many large images are there?
Making many exams in Moodle is a frequent situation in UNAM. I would suggest you to visit the UNAM Moodle admins and developers free site.