Osaka to Kyushu (ferry)

So there a number of ports in Kyushu. Eg Miyazaki, Kagoshima, Beppu

I would fly in to KIX. If I was to take a ferry should I take it from Osaka city or KIX?

Which port of Kyushu does the ferry goes to? How often and how much are these ferries?

My question includes buses that goes onto ferries.

The 2 ferries to Kyushu leave from the port of Osaka (Osaka Nanko) and not KIX.
http://www.japan-guide.com/e/e2355_01.html#f11

One ferry goes into Beppu in north Kyushu and the other into Shibushi in Kagoshima.

The Kansai Bay Ferry is the one that leaves from KIX. It just connects KIX with Kobe Airport.
http://www.japan-guide.com/e/e2033.html (at the bottom)

If you have a student ID, the long distance ferries give discounts. You just need to present the ID when you get your ticket. I think there's a different form to fill out at the counter also.

Both of the ferries are overnight, so I'd recommend getting at least a 2nd class bed. The "open floor" 2nd class is cheap but you could be with 30 people around you. I took this route once from Beppu to Osaka and had to sleep in the stairwell as it got so crowded with people getting on in Shikoku and they'd roped off all the general floor space.

Osaka-Beppu & Osaka-Shibushi
https://www.ferry-sunflower.co.jp/language/english/time/

You're driving a bus? Both of these are car ferries and take cars, buses, and trucks.