2005 379 c15 blown

I am the wife so bear with me. Im just a pissed off wife and im looking for others opinions. We bought a 2005 Peterbilt C15 379 from a guy that owns a company that hauls compost and garbage for the local landfill. Purchased for $40,000. It had a little bit of blow by and for a semi thats a little older and has i believe about 800,000 miles on it (i may be wrong but i know its not quite at a million) a little blow by is something that was bound to happen. Now a month later of only working it part of the day...It is on the brink of blowing up. Literally. It has so much blow by and it is burning a gallon of oil a day and also blowing oil out of the breather tube and dipstick. Brought it to Zeigler and they dont even understand how it is still running. Its pretty bad. Were quoted $40,000 for an inframe rebuild and however much more if inframe isnt possible. What pisses me off is how did this guy not know that the motor was on its literal last month...We were told it ran just fine. I am looking for some advice of some sort or even someone telling me im wrong to be a pissed off wife I undersand sales like this is on an as is basis but i just feel like a fool. We own our own business and this semi is our income. any imput is welcome. Just venting/looking for some advice. Thanks!!!!