I would like to open by saying that I will be praying for the success of your endeavor! It is truly a joy to attend a Missa Cantata or High Mass celebrated in the way that the council fathers intended. Might I suggest that you head over to the St. Cecilia Schola Cantorum website? They have a link to the Jubilate Deo booklet that was promulgated by, of all people, Pope Paul VI. It contains chants that every parish is required to be familiar with. ceciliaschola.org/ Unfortunately, in many cases one cannot expect to have a sung Mass with Gregorian Chant overnight.
Dr. Edward Schaefer of Gonzaga University, perhaps the top chant scholar in the nation, suggests first encouraging the priest and congregation to sing the dialogues that are notated in the sacramentary (ie: “The Lord be with you” “And also with you,” “Let us pray,” “The Word of the Lord,” etc.). This will bring about a very subtle but important change in the mindset of the priest and people. Eventually it will be possible to add the Gregorian ordinaries, and the crowning glory would be the singing of the propers as they appear in the Graduale Romanum. Dr. Schaefer has created, with the approval of the USCCB, a sacramentary which is fully notated so that every part of the Mass (yes, including the preface and the Eucharist prayers) may be chanted in English if desired. You can find that here priorypress.com/ There are also congregation booklets available that are VERY well-produced. There is the Latin chant in gregorian notation and english translation beneath that. The ordinaries are divided between the Schola and Congregation and organ accompaniment is available.
If singing the Gregorian propers proves to be too hard for your Schola (which it may for a long time. Our men are STILL trying to get used to the modality), a reverent alternative is the Anglican Use Gradual, which is approved for use in all parishes of the Roman Rite and contains the propers of the Mass in the King’s English set to Gregorian psalm tones. Quite pretty, actually, but not the same as the Graduale Romanum. justus.anglican.org/resources/pc/music/gradual/