Connecting grounds of DC-DC converter at one point
It's fine to connect like that, it just means you will have a non-isolated converter.

A single ground-plane should be fine too since this is just a power-converter board.  The usual reasons for a split ground plane (or more generally, a star topology of grounds) are where you have different signal-types in different circuit blocks and you want to ensure that ground-currents in one block do not induce a voltage in the ground of another block.

You may find that it's cheaper to buy a non-isolated converter though than to make one from an isolated converter.