AIR HANDLER & Compressor
Is this a heat pump or straight ac?
If it's a heat pump replace both units for max. effiency. Advantages of replacing now vs. later is you are replacing on your terms and timetable being proactive instead of being forced to do something on a moments notice. Also you can get 3-4 quotes for the best price and effiency you can afford. When it goes out, most times you are at the mercy of the contractor unless you are willing to be inconvienced for a week or two.
If ac, replace the ac system meaning outdoor unit, line set(copper lines) and indoor coil. Capacitors are relatively inexpensive and readily available. If they were weak, why not replace them now before they fail??? Why pay for another service call? I wouldn't worry about replacing the air handler if it is just an ac.
Things to think about: Purchase a 15 seer ac or hp(seer is an effiency rating for ac. the higher the number the higher the effiency). 15 seer is a nice effiency and you are not getting into the more complicated and expensive 2 stage compressors. An air cleaner for your home if you have allergies or other health issues you want to consider at least a merv 10 filter not an electronic air cleaner.
Hope this helps