Just to pick up on a few points made higher up:

Yes of course a melodeon CAN be played legato, and an accordion can play with bounce, but the natural features of the instruments mean that the melodeon is more suitable for bouncy English dance tunes, whereas if you want a smooth, less rhythmic sound, it will be more achieveable on an English concertina or an accordion (although many accordions are difficult to sing with). It's not wrong to play in a punchy, bouncy way if that's the effect you want (listen to John Spiers!) You might was well choose the instrument best suited to your purpose.

And if you do choose a melodeon for singing, I'd suggest you choose one with a choice of "voices", especially on the right-hand side (a Salterelle Nuage, mentioned already, would be a good example)so that you can get different effects and see which ones suit both your voice and your song.

As for singing keys: it's misleading to suggest that a certain voice will suit particular keys. It depends entirely on the range of notes in a particular song, and how that matches up with the range of the voice. Anyone who says "I always like to sing in F" or whatever really doesn't understand what keys mean. If you have a two-row melodeon, no matter what the keys are, there will always be some songs that you just can't manage comfortably in any of the keys (two major, two minor)available. If you do want to join in tune sessions or play for dancing in England, a D/G is the way to go.

Marje