That is a nice looking Yanmar!! My bet is it will be well worth the effort to gt it running right. Based on my experience with my Ford, running well it will be a real workhorse. My Ford is 20HP but since I rarely run it over 1700 rpm it's probably at around 16 hp at that speed. You might wat to check the glow plugs first. At arounf 17 degrees mine wants to run on 1 cly for a few seconds before the 2nd cyl fires(I think I have a weak glow plug).
I almost bought a Yanmar like that the day before I bought my Ford. One thing you might need to check is the front end.
They have a Wedge(with a threaded bolt) that links the steering arm to the kingpin that tends to wear and need replaced. I don't think they are too expensive. I found that out after I passed on one with wobbly front wheels.
There are many parts suppliers for Yanmar around the country.
Those paddle tires might take some getting used to, but I think you will like them. Remember the Japanese used these to run a tiller in flooded rice paddys. So a little Arkansas mud should be no problem at all.
Look forward to hearing more about this project!! I love the Diesels and am learning myself.