Replace entire A/C unit or only A-Coil.

[Carolina E.]

I have a single family house that is missing the A-Coil/Evaporator.  I called an a/c tech and, over the phone, said I should just replace the entire unit.  I am not sure if the condenser/compressor works.  It looks like it may be 10 years old.  I am not sure why the a-coil is missing, but should I have the condenser tested and see if it works and just install the parts I need?  Or replace the entire thing?  He also told me that the refrigerant it changing to a different type and better to just replace the whole system.  Thoughts?