Big tractors can soon run away with Cash, so my thoughts are look hard at what you are going to buy, on the older style tractors D9 series G/H, the parts are still very simple to find at very low cost, they are harder to work on from the spanner point, plus sometimes when I start stripping out engines that have been running for 50 years, the inside bits have had their day, but the parts as allready said are common to find at low cost, the High Drive iron do still use the Duo-Cone seals in the F/Drives, again the parts system both from CAT and Non OEM are well established, I can always find work on a High Drive tractor with my toolbox, plus there is the electronic faults that start to get you down unless your service staff are skilled in this area, the transmission shift (hard-soft) brake performance etc are all hooked up to a bit of cable?? its a well known fact that the old standard tractors have the ability to be rebuilt many times, but now with the engineering skills that can be taken to site like line bore etc, the High Drives are also very rebuildable.
The High drive iron looks better on sites, it has all the safety cabin bits and flashing lights, plus impressive to the Customer, the amount of fuel used per hour would be worth checking out though, as I have seen D6R tractors beat old D8H's on thirst duty.