Actually my Mk1 install doesn't require the earthing cables (if you add them it sounds terrible!) so this depends on your vehicle/install.

And apparently they need different software updates, but will all the updates for the MKII be available for the MKI too?

The code base for the two players is basically the same so every time new software is released for the Mk2 it is released for the Mk1 as well (unless the update is only related to Mk2 specific hardware).

Are there any other big differences, like lower voltage outputs on the MKI or lack of the equalizer, etc.?

The audio outputs are the same voltage on both players, and both have the same 10 channel parametric equalizer.

(list 6284, Mk1 S/N 00299 4GB blue [for sale]. Mk2 S/N 080000094 6GB blue)