The factory evaporators were aluminum and either corrode from the inside or the outside depending the expert. It is a 16-18 hour job to replace it, plus parts.
There are even more opinions on the R134 question. Mainstream opinion seems to be that you need to replace the compressor, drier and switches, manifold pipe, and expansion valve. That's at least $950 for parts plus labor for flushing and charging.
If the AC is in good shape, keep the R12. Consider the change only if you have replace a major component, and then only if you are contemplating a long term relationship with the car.