2003 Isuzu Rodeo Question Trying desperately to repair my ca

Engine Cooling problem
2003 Isuzu Rodeo Question Two Wheel Drive Automatic 77000 miles

About 2 weeks ago, my "Check Engine" light came on and I noticed that even after I turned my car off, the fans were still running for a considerably long time. Later on, I noticed that the gage was completely on the "H" side (rather than in the middle of C and H). My heater stopped blowing out hot air periodically. I went out of town for a week and it was not driven, but when I returned, something under the hood was making a slight squeaking sound and then, while driving to the gas station to buy coolant, smoke came out from underneath the hood. First, the mechanic said we needed a new radiator and a new thermostat and gave us a total of over $500. Then, he said that if we replaced the thermostat, we should be fine. Now, he's saying it could be the water pump. I'm so confused.