I would rate the Mitsubishi engine S4Q and S4S two of the best Diesel engines in world at those displacements, and applications. Iseki is better know for smaller diesel engines and is superior quality in the below 25 hp applications and has a reputation above Kubota and Yanmar in Japan. Although in marine applications Yanmar is in its own class above all others. While I confess to not knowing details of shibaura diesel engines, they never bubbled to top in my research and I would consider them average. I have seen some articles that makes me think Shibaura my make some Perkins designs under license, if that's true they my be a very good engine. I did a extensive investigation into those Mitsubishi engines before buying a U5030 in 2011, that model is an outstanding tractor at any price. They hit the bulls eye, and that may the best utility tractor you can buy, straight up, at the bottom of class 2 ranks. If you want to break into the bottom of class 2, this is the tractor, the U5030 with S4S. My two cents. HS