I agree with melodeon, above, where he recommended A/D or A + D one rows.
Haven't been playing old-time fiddle tunes for a while, but I used to have a number of Henry Reed tunes in my repertoire, in A on the D box.
One thing I'll point out, and someone hinted at it above but I'd like to underscore it, is the sound of the box could be just as important as having the "right" keys. Old time musicians can be a bit skeptical of non-string instruments and so having a great sound is a good start. For this, I think the sound of a four-voice one row can't be beat. I don't know if anyone does this, but it seems like the +5 Sagné boxes would be just the trick.
... and I've heard some great "old time" concertina, too, but this isn't the place for that!