Mozart's 12 Celebrated Symphonies are not piano pieces but orchestral works that have been transcribed. They showcase his mastery of form, melody, and harmonic invention.
N/A (Orchestral works)
12 Celebrated Symphonies is rated Henle level 8 of 9 — concert repertoire, roughly diploma level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 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 12 Celebrated Symphonies lasts about 60 minutes.
12 Celebrated Symphonies is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.