I would suggest a 2 or 3 Anglo for shanties. Dependent on your vocal range either c/g or g/d (a c/g gives more options when played in c, a g/d more when played in g). Unless you are very lucky you might as well buy a new Stagi or Gremlin 3 row. Good condition older Anglos are now quite expensive and repairs can add a lot to the price (a new set of bellows is around £150). Check out www.the-music-room.com or www.concertina.net amongst others.

The beauty of the Anglo system is you can learn it yourself by trial and error whereas the English or Duet systems (really designed for Victorian and Edwardian parlour musicians and the Salvation Army) are much more complicated and to my (biased) way of thinking illogical.

All the best

Steve