Auto transmission hard to shift gears

What is your question?

This is a used car I purchased and it drives ok, except that it feels like the transmission won't shift gears. The transaxle fluid is very dark (black)and has a very small leak. Do I need to rebuild the transmission/; and if so, can I get a ballpark figure? I have a 1993 which runs excellent; reason for which I bought this one. A local shop, Castro Automotive, recommended that I get rid of this chariot. Should I?