Gurk! What a strange thing for someone to suggest. You are quite right, just ignore the third row at first. But the "accidentals" row is not just accidentals, but reverses of important buttons and things of that kind. You will fairly quickly get impatient with the limitations of a 20-button box.
If money is an issue (and, concertinas being what they are, it usually is) and you really cannot go to a 30-button, then watch out for the 26-buttons that sometimes flit past on eBay and elsewhere. Much cheaper than a thirty-buttton, much more versatile than a 20-button, they often make a very good buy for a starter concertina.
Chris
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I'm honestly quite torn...I have NO background in free reed instruments, and haven't played anything at all in years, so the simplicity of a 20 appeals to me. I doubt I'll ever get "serious" about playing for audiences or anything. On the other hand, if I take to it, I'll want the 30....somebody just give me a free concertina so I won't have to make a decision!
Greg