That would be way too broad for me to answer, but I do have some suggestions: at the back of the Gleason Organ Method is an excellent graded repertoire list for all levels. You might also check out two books on organ literature: Marilou Kratzenstein (a good survey of nationalities and periods), and Corliss Arnold (a good, broad listing of organ repertoire).
I haven't played all of mine yet (and doubt that I ever will) but I just enjoy collecting music, too. My intention is for all my children to be better organists than I am so they can eventually play all the stuff I never could. I'll just get to sit back and listen.
Alone in my complete works of Bach, Buxtehude, and Krebs I have music to last quite awhile. No need to stray from baroque!
IMSLP is your best friend when seeking organ rep. Start with Pachelbel, Buxtehude, and Bach. Bach is probably the most difficult music written for organ that you can get for free. Marchand, Corrette, Nivers, and d'Aquin are accessible composers from the Classic French organ period. Franck, Dupre, Vierne, and Widor are more modern (I know Franck was Romantic era), and have written some very good music that is also available on IMSLP (except Dupre, who is too modern and much of his rep is still under copyright).
Some of my favorite works in my repertoire: (you can get all of these from IMSLP)
8 Short Preludes and Fugues (C Major and D Minor); Bach
Danket Dem Herrn; Buxtehude
Aria from the Goldberg Variations; Bach
Sonatina from Actus Tragicus; Bach
Suite Gothique (mvts 1 and 3); Boellmann
Adagio; Albinoni
Toccata and Fugue in D Minor; Bach
Grand Dialogue; Marchand
Communion; Vierne
Souvenir; Dupre
Petite Offertoire; Franck
Canto della Sera; Bossi
Summer Sketches (mvt 5 Evening); Lemare
Come Sweet Death; Bach (it has words, but is beautiful as an organ-only work)
Livre de Noels; D'Aquin (yes the whole book)
Livre d'orgue no. 1; Lebegue
Livre d'orgue; Clerambault
I think I'd better stop there before this becomes much, much too long although we may very well have passed that point.
The Royal Conservatory syllabus (the second link above) begins with Grade 7. Apparently they assume at least an intermediate level of achievement on the piano prior to commencing study of the organ, as do most of the organ method books with which I am familiar.
As to whether study of the piano is advised before studying the organ, there was a very long discussion of this on this forum sometime back.
For pieces for manuals only, I use many by Johann Gottlieb Walther; his choral preludes are published in three volumes by Breitkopf. A little expensive, but worth it. I got them from Amazon.de for about 40 euros a volume.
Others, similar choral preludes by Pachelbel and Georg Boehm. The collection of the choral preludes of J.S. Bach recently discovered at Yale (Neumeister Collection) and published by Bärenreiter. Toccatas and Ricercars of Frescobaldi and Froberger.
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