My experience with Yanmars is they are very well built and dependable. The models built for sale in Japan (grey market as they are called), seem to have quite sturdy paint jobs and usually show only surface rust after years of working in a very harsh environment.
Yanmar manitains a parts warehouse in Illinois and contray to thier public statments does support the grey's, as evidenced from parts ordered that have grey market parts numbers on them!
Yanmar also is integrated into construction, earth moving equipment, building domestic branded tractors, as well as marine grade powerplants for sail boats and off shore cruisers. If you live in a coastal community there is probably a certified Yanmar parts dealer in your yellow pages. They would not have clue about tractors but if you had a part number they could get it for you.
Also finding someone to work on a yanmar, if needed, is not really a problem. Any tractor repair house can and does work on them. Especially since Yanmar has been producing powerplants for JD for many years and for many JD models including farm tractors. Once you get them past the idea of it being grey market, it comes down to the same ole stuff they are already working on (in general).
Also, there are many fine dealers available on this board that are ready to ship any needed parts to your door.
Finally, If you shop for a Yanmar, try to find a dealer that believes in Yanmar and imports some of the better tractors that are offered for sale in Japan. That makes a difference in how much repair is needed when they get here.
Anyway my .02