If the leak is positively in the evaporator, I would replace the indoor unit. The cost of the newer R410A systems are quite high. If the outdoor unit is rust and corrosion free, it would be less expensive. It will be a risk knowing that the compressor is a 19 year old unit. The biggest problem will be finding an honest company willing to take on a small job. With any luck a knew air handling unit or the correct evaporator unit will match the existing dimensions. (not a problem for a good sheet metal mechanic) This reply is based on a simple box airhandler and not a furnace with AC.