The OP doesn't say what make of concertina he currently has, but decent 20 and 26 button ones, be they made in the England, Europe or China, are rarely of good quality (though I have encountered 20 and 26 key metal ended Jeffries and a 20 key Wheatstone Linota which are exceptions).

Almost any English-made or hybrid 30 button concertina will be of a far better quality than a 20 or 26 button, irrespective of whether the "extra" keys are useful at this time. For that reason alone, and if finances allow, seriously consider an upgrade to a 30 key (unless you happen to be the lucky owner of the above mentioned Jeffries or Linota).

Good luck Mathew, and if you find some of the crap posted on this thread to be a bit daunting, can I suggest asking for some advice at www.concertina.net