Thanks for the responses. This has helped me quite a bit...
I know the seller, he's my dad, and the mechanic has done some other work for him and he's always had good luck. I was looking for information to ensure that dad wasn't trying to 'give the farm away' so to speak, and trying to sell it to me too cheaply.
I know most of his costs, $500 for tractor and about $5200 in engine and trasmission work. Had the transmission not failed after the engine, he would have had a good deal going, as it is, he's lost his shirt on this one.
I figure that instead of financing a new NH 30hp boomer, I buy this from him for what he's got in it. He's not under, I've got more tractor than I need and still have something left to buy implements with. When it comes time for me to sell or trade it, it's got more time on the rebuild and worth more than salvage again.
Thanks again