There's absolutely no point in a basic one-row melodeon for song accompaniment, as it will only play one octave in one key, and only a few chords.

A simple electronic keyboard sounds as if it would tick most of the boxes for you. Now that Xmas is past, people may have upgraded and you could possibly pick up something suitable at a charity shop or a car boot sale (weather permitting). But beware of the very cheap ones that run on batteries, as they could cost a lot to run. Either find something that interfaces with your computer or a simple Yamaha or Casio keyboard wthat plugs into the mains.

I'm quite excited because I got a Yamaha keyboard for Christmas. I just love messing about with the chord accompaniments and the different voices. I have no intention of using it to perform in front of others (my daughter has told me sternly that my "Soul" version of Baa Baa Black Sheep is not going to win the X-factor) but I'm having huge fun with it. I also think it will be useful for working out harmonies and chords, and generally getting to understand a bit more about how music works.

Marje