To bad you don't have someone to help you with the work. I personally would install the input and output sensors and see how it works, but I would do the work myself. If works properly I would then change tranny fluid and filter. The reason's I would try it this way is that the sensors are $16.00 a piece and the solenoid is $209.00 according to the napaonline.com.
The dealership could very well be correct with their diagnoses but I would get it in writing that the solenoid will fix the problem.