Mixbus is a separate program. You cannot add it to Ardour. They are very similar since they share a lot of code. The main difference is Mixbus includes advanced DSP in the mixer section. Similar results can be had in Ardour through the use of third party plugins.
Regarding hardware, it takes an audio interface that is compatible with your operating system. All things being equal at the source, the quality of the results will be in line with the quality of the interface.
I am not in a position to comment on the SSL Nucleus 2’s compatibility with Ardour, but…are you a professional sound engineer? It’s a pricey device that seems well beyond what most beginners require.
I’m pretty sure Indamixx was discontinued a long time ago. I think it was just a laptop with Linux audio applications pre-installed. Ardour added to it, not the other way around.
ArdourXchange allows a user to import sessions from certain versions of Pro Tools into Ardour or Mixbus. It is another completely separate program that is useless unless you are trying to accomplish that specific task.
The SSL Nucleus 2 is primarily a control surface. It is expensive - in my opinion, over-priced and oddly designed. It is a very nice feeling control surface, but if that’s what you wanted there are equally nice cheaper options. You will not be able to use it on Linux as an audio interface since it uses Dante which still has no working drives for Linux (it still works as a control surface). Details of Ardour’s support for the control surface are in the manual at http://www.manual.ardour.org/using-control-surfaces/devices-using-mackielogic-control-protocol/ssl-nucleus/
Even with a Nucleus, you will almost certainly want a different audio interface for actually recording. There are many available and google is your best best to find some. You need to understand how many channels you need/want, how many (if any) mic preamps you need and so on. These are not questions that are specific to Ardour or even DAWs but apply to all attempts to move digital audio in and out of a computer.