OGGIE,
I did not compare the kubota tractors, I however did compare the New Holland TN90, and found the Deere to be priced about $2500 less. Not exactly sure that I was comparing apples to apples, as some of the specs were different, namely the TPH lift capacity was much higher on the NH.
The larger Kubotas did not seem as popular around here, so I did not even visit the nearest orange dealer. I am sure that they are good products if they are anything like their smaller counterparts.
As to mower size selection, I think that your 5420 will pull most mowers, and as such your size selection should be based on how much time you consider acceptable based on how much you have to mow, and how wide you can get away with for getting in between all of the trees and other obstacles on your property.
The cost difference between the 7' and 8' cutters is most likely because the 8' cutters will generally have two shorter blades as opposed to the 7' single blade. This adds two more gearboxes to the mower, and can substantially increase the cost.
If it were me, and if I had the clearance, for mowing 75 acres, I would not look at anything less than a 10' cutter, and would probably consider a 15' batwing mower. Perhaps something like the BUSH HOG® 2615L Flex Wing Rotary Cutter.