Check out anew version of Mozart's letters from Italy is online here:
The website offers access to the letters in English, Italian and French, and of course the original German version plus its modernised German spelling
For the majority of the letters, both the holograph and its diplomatic transcription are available (left-hand side menu).
Mozart's scores are linked to each work, like here:
The letters are fully annotated by Cliff Eisen , Academic Director of the edition and one of the most reputed Mozart's scholars.