Generally, if air conditioners are manufactured before 2010 your old unit is probably equipped with R-22 refrigerant. The Environmental Protection Agency banned the manufacture of air conditioners with R-22. Production levels of R-22 have decreased pushing up the price per pound of this almost obsolete Freon to anywhere from $75 to $100 per pound. If you are losing refrigerant and are having to have R-22 added to your system, then at the very least replace your system with the newer ozone friendly refrigerants.
What to Replace
Most A/C replacement means changing the furnace also because typically the heating and cooling system share an old and inefficient blower motor that will reduce the efficiency of the new air conditioner.
Under certain conditions where the compressor, reversing valves or deteriorated condensers are too costly to replace you could consider just replacing the outdoor unit with the dry shipped nitrogen charged outside air conditioner if you don't want to bear the expense of replacing both indoor and outdoor with the newer ozone friendly type refrigerant.
Although the best scenario is to replace both pieces of equipment your amount of funds available will most likely determine your decision.
Matching Systems and Other Options
Whenever your outside condensing unit goes out most genuine or reputable companies recommend that you replace your indoor unit air handler at the same time, because the two units must be matched so that your new outdoor unit will work at maximum efficiency. Improperly matched units will shorten the life span of your new unit's compressor, which could be a serious and costly mistake.