If it was me I would do it all, but I am really a maintenance freak so take that for what it is worth. To me fluids are cheaper that replacing a trans or motor.
I think this reply says it all. On this forum you will find only the people who care about their equipment and believe in maintaining it. They have done the math and feel it is the right choice.
There are many threads that ask 'should I change the filter, or both the filter and oil?' or, 'which oil is better?' and the answers are nearly always the safe one : Do what you won't regret, and use the best Kubota has to offer.
If the question is 'which course of action is good enough?' That is often a personal decision. Would anyone who offers advice that is 'good enough' cover your costs to re-do the work?