These Kubota engines can be exchanged for @ 3200.00 If you did all the leg work yourself likely 500 to 1000 less. Most people consider 5000 hours the life, Not 10000 like many of the larger diesels. And depending on the application many loaders have consideable wear in 4000 to 5000 hours too. Anything on them can be rebuilt. I don't know if they are sleeved or if you just bore them. If there are not sleeved the block can only be bored once likely, then they would have to be sleeved.
Good looking 743's are worth @ 7 or 8K + or-
Most diesel bobcat never dip much below this if they still run and don't appear to be on there last legs. It really does come down to how much work it needs to become a good loader again.
If the cost is higher then buying a good one, then fixing may not be the right thing, especially if you may want to sell again sometime.
But if you plan to keep it a long while and it will meet you needs, then you might be more inclined just to pick one and fix what it needs so it will work for you or give it a total overhaul so you know its good. As long as you keep it you can be sure you will get your money back out of it with a little care and maintenane. Selling and recovering your dollars can be a bit tougher though.
As for the hours though thats only a guide line. look how much slack the loader pins have and that will give you a good idea as to how long and or hard of life it has already had. (note bucket dump cylinder pins are the first to go, expect 1/4" of slack by 3000 hours. If its 3/4" it has done some hard work or never seen any grease.
Will the boom and bucket stay up or do they leak down in a few minutes. Blowby and cold starting of the engine, bent loader arms with non factory welds, bent cab, back door etc all give you a idea of how its life was.
Ken