What 5 tonner

I am contemplating upgrading/retiring the old 4 ton Dyna and need advice on way to go. Been handy but lacks power for highway work. Isuzu seems the safe brand on account of affordable spares and known reliability. Living at 1,000 metres means that I want something that doesn't get a heart attack when it sees the mountains and does 100km into the wind easy. Also aircon. Could go NQR450 but then the FRR & FSR have some room in the cab which is desirable. I am confused about difference between FRR and FSR as they overlap. Also engine choices within each model. Obviously pre egr & computers. Needs to be light enough to poke around paddocks.
Any thoughts?

I drove a nqr 450 loaded from Newcastle to melbourne once it was no rocket but acceptable fuel consumption was average as well ... The older fsr & ftrs ( before the ohc 6 cyl arrived? We're nice trucks & as you say a big enough cab

At this rating you are choosing between the biggest of the 4 cylinder Jap models or the smallest of their six cylinder ones. I reckon go the bigger ones ( F series ) as they will do the job better and components aren't working as hard. Plus more room in the cab and (usually ) air over hydraulic brakes.