For my mini loader project I have the choice of a Kubota D6oo or a Briggs 18Hp, which do you think would be better?
Depends, the diesel would likely be more fuel effecient than the gasoline engine. The Kubota would require a cooling system but from a reliability stand point the diesel would probably also be more reliable than the gasoline engine would be.
I have an Izusu diesel engine I am going CadTrac project. It is a 34hp if I remember correctly. The D600 is rated at 14hp from what I can find on the net, in a limited search.
A 14hp engine should be able to run a 5GPM pump at 2000PSI, an 18Hp engine should be able to run 6.5GPM pump at 2000PSI.
If you can deal will the cooling system than the diesel would probably be the best choice. But if you do not want to deal with the cooling system than use the gasoline engine.
Just my two cents worth.