pulling question bank out of Moodle

I have 3 different Moodle databases, each one represents a school, I want to built an external question bank that can be accessed by these 3 schools as follows : when a teacher creates a new quiz, the quiz editing page will look like the same of Moodle's one, the same question bank, but this page actually reads its content from an external question bank system, the teacher adds, deletes and updates questions in the bank as usual, the only difference here is that the questions come from that external question bank database and not from Moodle's database. what are the important points to consider before starting to build such question bank ?