2001 Ford Windstar Valve Job?

I have a 2001 Ford Winstar that has started developing problems and I have no idea what to do. The check Engine light is on with Lean Codes (I dont have the Numbers). About a week ago while driving the engine kind of choked up and lost all power. I thought that I heard a pop and then the engine took off again. But under less power. The engine was running very rough. The exhaust was very stinky. I took the van to a local certified mechanic and got it back yesterday. He changed the spark plugs and says that one of the cylinders is dropping out. During compression check he said it registered 70 PSI and then a second check registered 90 PSI. He stated that I need a valve job. He said it was an exhaust valve because he put a rag over the exhaust pipe and it sucked back into the pipe a little. He also said that it is possible that carbon buildup on the exhaust valve may burn away and it might run fine. The problem that keeps coming to mind with me is that this was not a gradual loss of power. This happened at one moment. Can a valve go bad that quickly? Also what are the ramifications of driving it to try and "Clean it out"? Can I cause more damage to the engine. The van runs better since he changed the plugs but it still runs rough at low RPM and lacks power at low RPM. I have filled up the tank and put some Gumout Fuel System cleaner in the tank to try and help the "CleanOut" situation. How can I be sure that I need a valve job? I would hate to burn that kind of money for work not needed. It already set me back $200 for the plugs and diagnostic.