Mozart's 'Adagio' in D minor is a deeply moving and introspective movement, characterized by its poignant melody and dramatic harmonic shifts.
Achieving the expressive weight and dramatic tension requires sensitive control of dynamics and articulation.
Adagio in D minor is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7. In the RCM system it corresponds to about level 8.
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.
A typical performance of Adagio in D minor lasts about 5 minutes.
Adagio in D minor is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.