Contact Sound Deadener Showdown for info on how to properly sound deaden a cab. Hint: covering every square inch in DynaMat is NOT the answer. Different products are required to attenuate different frequencies and when installed properly, these products will significantly reduce time, weight and money, when compared to the 'cover-it-all' philosophy.
Cabover trucks ride like ********. That is the plain truth. Commercial truck seats, suspensions and tires are simply not designed with comfort in mind. You will spend a small fortune getting a COE to ride like a $5,000 used car. Fox and King make decent dampers, but quality and customer service has suffered in recent years, according to contacts in the industry. An MDT has a lot of mass to control, whether that is a camper box or tire & wheel combos that weigh 250 - 350 pounds. Check out Reiger, as that is what most of the Dakar trucks are running.
Suspension seats are a Catch .22. A fixed-mounted suspension seat allows you to move in the seat, leaving you 'unrestrained' in a crash. Air ride seats are even worse. Seats and tires are not suspension, they are Band-Aids for poor engineering and design.
Consider a high end suspension seat from Mastercraft Safety, Sparco or Recaro if you are going down that path. Alternatively, you can buy an alloy or carbon shell that is too large and have a company like BCSI pour 3-4" of foam, contouring the seat to your body. Total cost is in the $500 - $700 range, including the seat.
Fusos, NPRs and KWs are commercial trucks made to carry heavy loads for hundreds of thousands of miles. They are not designed to travel at 80 MPH, drive cross-country or be comfortable. If you want comfort, buy a used LX570 and use the money you save to have one hell of a good time. If you are certain that you want a large 'expedition vehicle' be prepared to make compromises or spend lots of money making it into what you want it to be.