If you intend fitting a combined unit, don't go too small. I've got a Lowrnce HDD5 and was always switching between plotter and sonar as the screen is too small in split mode. I therefore fitted a Lowrance Elite 7xHDi. And use the HDS5 as plotter only.
As Rob said, the Dragonfly can't be interfaced with your VHF radio, unless they've brought out a later model.