1991 240sx: What are the symptoms of a dying Distributor?

[grachman]

these are my symptoms:
-car sounds like the muffler is going. I guess it is missing . has been doing this for at least 5 months.
-rich at idle
-car started showing the code for speed sensor about a month ago(speedometer works)
-car is now hard to start in cold or hot conditions. disconnecting battery seems to help. I'm getting code21 now-primary ignition signal circuit fault
-car recently died while driving. restarted after disconnecting battery cable.
-replaced ignition module and ignition coil

Are used distributors worth it. They sell for $200 in my area. $400 for a remanufactured one.