Johann Sebastian Bach's '16 Konzerte nach verschiedenen Meistern' are masterful transcriptions of concertos by other composers, adapted for solo keyboard. They reveal Bach's profound ability to infuse existing works with his characteristic contrapuntal complexity and structural integrity.
The main technical demand is the precise control required to articulate multiple independent melodic lines simultaneously.
16 Konzerte nach verschiedenen Meistern is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
16 Konzerte nach verschiedenen Meistern is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.