Recently, our schola at the parish sang a Requiem Mass in the ordinary form, and the music question was wide open. We chose Gregorian settings of the entrance and communion. For offertory, we relied on a resource that has become essential for our weekly work: The Simple Choral Gradual. It contains four-part settings for all the propers of the Mass for the entire year.
No matter how much I write about this book, I can't get over the sense that it is underappreciated. Especially for these special Masses, the composer (Richard Rice) provides outstanding music that is not difficult and yet expresses a unmistakable liturgical solemnity. The offertory for the Requiem Mass was a great example of this.
A disadvantage that this book has is that there are no score-displaying youtubes of this book, unlike the Simple English Propers. If we had more recordings, I feel sure it would be more well known than it is. The fact is that this resource is something that no ordinary form schola can do without, provided the schola is interested in singing the liturgy as versus just singing something at Mass.