Hello Tom
There is a secondhand JD 2600 on BFF at the moment, you need to know whether the auto steer is unlocked for JD's only, or for ATU's and what level of correction it is unlocked for. £500 each for secondhand ATU's and it could be on autosteer for under £3.5k. If you need to unlock it for SF2, or to use my RTK it will cost more!!
On the question of what to spend for 150hrs drilling a year, the brass tacks answer boils down to 'not much'!! But the quality of life answer, for an owner operator who goes home to his family not as tired, or fatigued, from a more relaxed day on the drill, is a different answer!!
As for what to do with tractor numbers, as for the previous paragraph how you want to run your own business has more of a bearing on the answer than harsh financials.