"If you do the maintenance on one, could you reasonably not be exposed to the exspensive coil replacement? I have heard that could be a $6 to $7,000.00 dollar repair?"
This seemed to be the OP's main concern. I have NEVER, in over 20 years, heard or had the water Heater coil fail from standard use. They HAVE failed when the Aqua Hot has not been winterized correctly and the coil freezes. Once that happens, Aqua Hot does NOT sell that part and you must send the complete unit back for rebuild. THAT is the 6 to 7 thousand dollar cost figure. This has NOTHING to do with maintenance. Doug