There are two basic terms - English and Anglo.

English plays the same note on push and pull when you press any single button.

Anglo plays a one note on push and a different one on pull. Anglo has two rows of buttons each side for two musical key pitches and some have a third row for sharps and flats.

English is easier to learn basic accompaniments on but harder to play fast tunes.

Anglo is the favourite for fast Celtic Jigs/reels etc.

Any old concertina in good condition and bargain price is worth buying. You can resell, swap or trade. Just check every button sounds a note except for the "air" button. Close the bellows and then hold the concertina vertically in one hand. If gravity opens the bellows and they "drop" open they are leaking.

For Shanties and sea songs you need an English. Basic accompaniments are quick and easy to figure out especially as you will only probably play to suit whatever key you sing in so you will tend to hover round the same set of buttons.

There are also "Duet" systems but they are not for the novice.

Good luck Mariner. Go for it. They are great fun in spite of the jokes. Anyway. concertina players tell jokes about banjos.