The Hohner Student III has 25 keys and 24 stradella bass, running from A to Ab, with maj, min and fundamental buttons. This would be a good accordion for someone who wants a lightweight but serious instrument, or for a learner. When restored (as this one was) these instruments make a marvellous sound because of their old reeds, which are mounted in zinc plates (modern reeds are normally mounted in aluminium plates). Please see the video demo.
Small piano accordions really should not be overlooked, especially if you want to play in folk sessions and want to carry the thing around. The Hohner Student III has 24 bass and 25 keys. I had two of these little instruments and completed a complete rebuild of one of them. This entailed dismantling the keyboard, replacing the messed up finish on the keyboard, re-facing all the pallets, then overhauling the reeds (clean, revalve, tune etc), sorting out the LH mechanism, fixing the bellows and so on.