I hope you're draining the water overnight if it's getting close to freezing. The longer this goes, the less likely the previous owner has any obligation to help out, if he did when you first added water. If I was the seller, and I had this thread describing how it was operated, I sure wouldn't want to do anything either.
Yes, change the oil as soon as you get it hot. Go to CAT and get SOS oil sample kit and take a sample when you change the oil. Run it with rainwater until you know it's holding coolant, but drain it after use. Watch the valve cover and/or dipstick for condensation/chocolate. And see what the oil test shows. Make sure the temp gauge is working, and confirm that it's full every so often.