Air seats are going to cost a fair bit - around $2K per side as a minimum. Unless you get OEM Unimog air seats, you will need ones with integrated seat belts, and then you need to build stronger seat mounts. The OEM seat belt mounts include the thick steel bar across the back with the which is attached via the cloth strap to the reinforced bracket on the rear wall of the cab. If you fitted air seats with integrated seat belts, you have to reinforce the floor and fit heavier duty bolts - the little OEM M8 one are not good enough you need M10 Gr10.8, You will also need the Mod Plate, which requires an Engineer to do all the calculations to prove it is strong enough, Mine ended up with 6 M10 Gr10.8 with 75x75mm 5mm backing plates.
I fitted Range Rover seat, very comfortable but a bit big, so we might be fitting some Recaro seats in the future. We didn't like air seats, to bouncy. I drove a couple of trucks with air seat, was difficult to control the throttle on rougher tracks. The suspension on our truck is quite soft, and with the soft Ranger Rover seats, pretty comfortable off road.
Mog Central and Unidan sell air seat kits, but they are not cheap.