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.

Just been messing with my toy melodeon & accordion on a couple of E. Trads (Lovely Joan & Miller of Dee) & I would prefer them to sing with to any electronic keyboard no matter what they're limitations might be! This doesn't make any sort of luddite as 50% of my musical output I do using computer, sequencers, soft synths, controllers & (my favourite instrument of all) Ableton Live - but when it comes to traditional song I feel as if I need to be singing against something a lot more corporeal than a vibrating cardboard / plastic cone. My Electronic Shruti box would seem to be the exception to this rule for reasons I can't quite figure out, maybe because the cone only gives a drone.   

Me thinks an Indian Harmonium would suit you just fine, Pip - there's some nice portable ones around, foldaways & travellers which sound very nice indeed.