Nothing wrong with developing a prototype on your own, but it makes integrating later double difficult. So give Moodle a second chance: it has lots of options:
Reason one:
it is possible in Moodle to name an assignment "not on the computer" but still make a reservation to store the grades/marks for that external assignment in Moodle
As soon as the teacher brings in his marks, the student can see it in his privat scores-box (Coourse by course, no central overview in Moodle)
Reason two:
In Moodle it is possible to develop your own grading scales and use these for grading:
So you can create there your two scales 1,2,3,4,5 and pass/fail
Would be great if these grading scales were availble for all Moodle Moduls, there you have a point.
Grades for tests are in Moodle automatic, you can do corrections in the test by changing the weights and do a recalculation.
Wish you luck