The best way to go about finding such a composition which is technically still accessible to a relative beginner and musically interesting enough to keep you interested for a long time is to choose a piece by classical organ composers. By classical composers, I mean that these are masters whose works have stood the test of time. So anything you will find written by a major organ composer will be worth playing and worth practicing for a long period of time and you will not feel bored.
On the other hand, even in these classical pieces you will find a high number of places which will be technically too challenging. So what you have to look for in these pieces are texture, rhythms, and key signatures.
The texture has to be simple enough and rhythms have to be not complicated and the keys should be easy to understand and to play. Let's consider each of these three points in turn.
The simplicity of the texture means that it's best to choose the piece which has only a few voices. Try not to choose four and five or even six voice compositions if you are a relative beginner. The most number of voices for you will be three - one for the right hand, one for the left hand, and one for the pedals.
The next thing to consider is the simplicity of the rhythms. In other words, the note values have to be not complicated and you should not see any syncopations and smaller note values such as sixteenths and thirty-seconds. The smallest note value has to be an eighth note or an eighth note triplet.
Lastly, the keys have to be simple and easy to play for you. If you are a beginner, try to avoid pieces which have more than one accidental next to the treble and the bass clef.
Consider my tips in your practice and apply them when you choose the piece to play. If you choose wisely, you will be able to learn and master it very effectively.