Hello, I was wondering if someone can give me some advice on what to do with my broken transmission?

The vehicle is a 1995 ford ranger with 257,000 miles. Its been in the family for a while. I just got it 2 weeks ago since my sister got a new car so I was next in line. I loved the truck but I knew something was wrong with the transmission because it would shift hard sometimes. When we first got the truck back in 04 the transmission was replaced. The truck then blew a head around 210,000 miles. Me and my brother fixed it for my father. He changed the oil on the motor every 3000 miles since then but he forgot the oil on another crucial part of the truck. Yes, the transmission. He NEVER changed it! a few months ago I checked it and It was DARK brown and had a weird smell to it. We asked our mechanic what to do and he said there was nothing we could do because the transmission was already adjusted to the bad oil and giving it good oil would just make it s*** itself. Therefore we knew it was going to go out. Well a few weeks ago it gave out going up a hill with HIGH wind. I was driving when it went out. When it happened there was no noise. It simply just didnt grab anything.. I pulled over and we got it towed. Its been sitting outside and I need a new car but my father wants to fix it. I tell him theres no value in the truck anymore since the motor has tons of miles and the cost of the transmission would cost more than the actual truck. Anyways, I found it weird how no noise was made when it "broke". I was talking to my cousin about this and he suggested flushing the trans fluid anyway and put in new oil. It doesnt seem like a bad idea and who knows? right? But maybe theres someone out there who can maybe help me out. I hope I gave enough information about the transmission. Thanks for the help!