Bach's "3 Chorale Preludes" are masterfully crafted pieces that explore the contrapuntal and harmonic possibilities inherent in Lutheran chorale melodies. They are revered for their contrapuntal ingenuity and profound spiritual depth.
The main challenge is to achieve clarity in the contrapuntal lines while maintaining a cohesive and expressive whole.
3 Chorale Preludes is rated Henle level 4 of 9 — intermediate repertoire, roughly ABRSM grades 5–6. In the RCM system it corresponds to about level 5.
You should be comfortable at Henle level 4 — meaning you can already play level 3–4 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 3 Chorale Preludes lasts about 12 minutes.
3 Chorale Preludes is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.