John, I don’t know if it warrants replacement under MY business standards, but it is a mess for a new unit.
The evap coil makes water as part of it’s job (absorption of latent heat results in water vapor condensing). I see this as crappy equipment. It should hold up better than this.
My thoughts of the unknown, is that there is a problem with airflow. The coil is getting too cold making the coil frame and adjacent materials to become too cold, causing them to condense and rust. The more air flow, the warmer the coil tends to become. It think it is just getting too wet in there.
To test for this, I would have the return air static pressure measured at the unit. If excessively hi (actually low in the return), this indicates return restriction from improper size or design.