A piano accordion can be an excellent choice because they are very widely used and are played in a similar style to a piano or keyboard instrument.  This enables a fast start in playing and the bass can be added in.  The bass notes are grouped so that for a given key the bass notes and chords are usually very close together.  A mark on several bass notes enable you to locate the note by feel.  The preferred professional size is 41 treble notes and 120 bass.  However smaller sizes can be supplied to suit customer requirements.