I'm learning too, that the evaporator is a common 850 problem, probably the weakest part of the car.(also check often for oil leaks, & look up posts on the flame trap issue) The dye tracer in the AC is the best and only way to really pinpoint leaks. If it were me, I would do what I wish I had done when my 240 needed AC work. And that is have them show the exact location of the leak and make sure it is not a fitting leading into the evap. or something more minor. On my 850, It was just charged after being very low, and I'm hoping against hope that they do lose freon with long term use, without leaks. Fingers crossed for both of us.
Good luck
Have fun