Usually the next stop announcement system is driven from the destination boards.
With Hanover Displays I believe you need a HTC unit fitted that is controlled by the standard destination controller, so you really have the simplicity of installing the unit, installing speakers throughout the bus linked to the HTC, then installing your next stop information screens, again connected to the HTC.
With a Mobitec you can't use the commonly fitted ICU400 or ICU402 controllers, and instead would have to purchase an ICU602 and if the bus is fitted with a 5 port communications connector it would need replaced with an 8 port connector to allow the interior next stop screens to be added.
Equipment wise they're simple to install, albeit not cheap to purchase. The more time consuming parts are setting the routes up to GPS co-ordinates with their relevant voice announcements, and recording the voice announcements. It is possible to purchase text to speech software to create the announcements but it's more common for companies to hire a voice actor/actress and have them recorded in a studio - a time consuming task!