How much is to much blow by on a 7.3 diesel?

I'm thinking of buying a 2000 ford f250 4x4 with a 7.3 v8 diesel, my question is the motor has 240000 and the owner says there's some blow by, i've heard that these motors can go in the hundreds of thousands of miles, now this truck is amazingly clean in and out and runs strong and the price is right at 6500.00 should i be concern about this slight blow by.

Hard question to answer without seeing it. If its just some fine smoke maybe coming out a handswidth, it is probably ok. If its puffing out or making it halfway to the open hood, it would be way too much. That's about as close as I can tell you from here.