1996 Nissan Sentra needing a new engine

Engine Mechanical problem
1996 Nissan Sentra 4 cyl Two Wheel Drive Automatic 200K miles

I bought this car new in 1996. I have kept up with regular oil changes and stuff. I had a check engine light on last year, I took it to 3 different machanics till finally months later they found it was a sensor and dry rot hose. It seemed the car ran like new for about a month, but then my husband checked the oil but forgot to put the oil cap back on after adding oil, I was driving for over an hour before I stopped and noticed smoke and smell from under the hood. So I replaced the cap, filled up the oil and drove another hour to get home. I then drove the car again in a few days, but the smoke smell was making me sick, so a friend said he would clean the engine, and he used a car wash, well when I went to drive home it had no power, and was running so rough, so I took it to a machanic and he found water in the spark plug area, and blew it out with water. The car ran great home. A few days later it wouldn't start, I had it towed to the shop and they tell me the motor is blown, I need a whole new engine at 3,000.00. My question is which event caused this motor failure? Was it my husband forgetting the oil cap? Was it the friend who washed it in the car wash? Or was it just a coincedence that it happened during these other events? And is 3,000.00 a reasonable price to replace the motor?