I have the rear engine version of this bus, so I have looked into some of this. IC clusters of this era are notorious for failing. While you can get a replacement cluster, I don't know of a drop in non-OEM cluster. I do plan on eventually getting a Bluefire adapter and mounting a tablet my dash to display gauges (
https://bluefire-llc.com/store).
For the seat, you could probably use one from an RV, but I don't know if the holes would line up. You may have to drill new holes in the floor. If you have an air ride seat, you could also look for a used seat from a semi.
I would think that adding A/C dash would be challenging, but with enough money, anything is possible. In my bus, I have an evaporator/air handler above the windshield. Since you already have A/C, it may be easier to relocate the evaporator (the part inside the bus) to above the windshield if it isn't there already. Instead of dash air, I plan to create some sort of vent to direct some of the air more down at me.