You found someone who is selling transmissions for a grand with a year warranty on this truck. Hmm I would be very leary of this. The brakes you were saying are bad, how bad? Calipers, shoes, new rotors can be very pricey. 240k, do you know if the head gasket has blown? GM engines are known to have the head gasket blow between 125K, and 180K. 300K is about the life of and engine, rings, valves, etc become really worn, break etc. Cost to replace all that, is why folks usually scrap em, or new crate engine.
Mine blew head gasket at 128K, I got lucky and it didn't eat my thrust bearing bottom of engine. I was told by GM mechanic, that usually happen when prolong driving after head gasket blows. What I would do in your shoes, is add up the costs the have this truck up n running/driving. Then look at the "Good" condition in Kelly Blue Book. See if the effort is worth the value you may get out of it if your planning to flip. Perhaps parting it out, selling engine, 4x4, and other parts that work, may bank you more$$ than paying out to fix it. My two cents