Finding a suitable repertoire

Sam writes that his goal is to become a church organist, and perform all organist functions from accompanying the congregation and choirs, to playing the liturgy (Lutheran and Catholic), and also be able to improvise.

The three things that are stopping him from doing this are lack of strong
foundation, repertoire that will make him progress, and good practice techniques.

Sam is right about the importance of having a strong foundation to your success in organ playing. But actually overcoming his other challenges (finding graded repertoire and applying good practice techniques) will most likely build his foundation for him.

So what you need to do, if you face similar obstacles to Sam's is always remember how you can find a suitable repertoire. This is done by the following way:

A good repertoire should be a quality music and it should make you stretch (but not too much). It shouldn't be a piece which you can sight-read without any difficulty. On the other hand, you should pick a piece which you can imagine performing in the next few months (not years).

So what are the basic guidelines in determining whether or not your piece is suitable for you to learn? It's actually very simple:

If you can sight-read separate voices of this piece at a tempo which is 50 % slower than a concert speed with 5 mistakes per page or less, then you can do it. Anything less than that will be a long shot for now.

Sight-reading:
Menuet-gothique (p. 4) from Suite Gothique, op. 25 by Leon Boellmann, French Romantic composer and organist.

Hymn playing:
He Was Not Willing