I had the pleasure of personally knowing the ex Australian Army officer who worked for the truck and bus division of Daimler Benz (as it was then known). His main job was to convince the Australian government that the Unimogs were the best thing for the Army; he was very successful. I went in a couple of the Unimogs and the first thing I was told to bring, was a pair of ear muffs. I did and they were needed for highway work, still are if you are talking about Ex army Unimogs.
Sound deadening of the cabin certainly helps. Changing the final drive ratio by however means, helps also. I'm pretty much certain you will need an air conditioner fitted, things can get decidedly warm. As for the seats, contoured wooden seats may be slightly more comfortable.
I really looked hard at the ex army Unimogs, good value for the money, or so it seems. For fantastic 4x4 go almost anywhere capability with living quarters on the rear; great. In the end they were discarded as something useful to us, we are retired, in our 70's and something slightly more modern with more creature comfort in the cabin pushed them off our list.
The FSS and FTS are quite different vehicles, as is their base cost. Essentially the FSS is an NPS on steroids. All of the offerings from Isuzu run duals at the rear, something which I didn't like, so before you start with either of them you are looking at an engineering issue. Not a big deal, but changing wheels, tyres and possibly suspension, isn't cheap; even if you do it all yourself with the aid on an appropriate engineers certification.
Taking all three of the S models offered by Isuzu, NPS, FSS and FTS, best value for dollar is the NPS, last price I saw was $68,500 ex tax for the cab chassis. It is of course the least equipped, but it isn't too bad.
The F series immediately jumped up to $130,000 for the FSS and I never even contemplated the FTS. The NPS and NPS are part time 4WD, the FTS is full time 4WD with a centre differential lock.
The FSS runs pretty much the same 5.2 litre engine as the NPS, but instead of 114kW it pulls 158kW. By comparison, the FTS runs a 7.8 litre motor and presumably an appropriate greater fuel usage as it produces 191kW.
Major differences between the NPS and the FSS are, bigger single cabin for the FSS, which allows for genuine Isringhousen seats, which are heavenly.
Larger wheels 17.5 versus 22.5 for the FSS.
With slight overhang of the chassis rails, the NPS should allow for a 5m long box. The FSS should allow 5.5m to 6m to be built on the rear. The idea is to keep whatever you have as short as possible. The FSS is the largest Isuzu that can be kept under 7m in length in a finished form.
Turning circle of the FSS is between the NPS and the FTS, with the NPS being surprisingly nimble.
One aspect you may not have thought of is, weight carrying capacity. If you are going to run super singles wheels, then the FSS and FTS will lose a bit of carrying capacity as their rear axle carrying capacity will be reduced.
I have in-laws running an older MAN TGM 13.290 on a farm with ex factory super singles, wonderful vehicle, but expensive as anything. It would be my choice of vehicles.
We have an NPS.