which key harmonica to buy?

I have a song alternating between Dsus and Emaj, then goes into Dmaj, Amin, C. Which key do i go for?

I’d try it with a G harp and see how it goes. If that doesn’t work I’d try D.

Your song starting with a Dsus indicates that it’s in the key of D but the C chord is unusual for a song in the key of D. All the notes in your chords conform to the Key of G and with a G harmonica you can play key of D in 2nd position. If a G harmonica doesn’t feel right then look at the D harmonica. Playing around your c chord will be a bit tricking but the C note can be easily hit with a 3 hole draw bent 1/2 step and you can also find a C on the 6th hole overblow.

You might also find that the song works with a harp in C. It has all the notes in all your chords as standard blow and draw no bending required except for the 1st octave where you always have to bend to get a complete scales and your F# which is a 5 hole overblow for the second octave and a 9 hole blow bend for the 3rd octave.