There are so many variants on the DC-3 theme that entire books have been written.
The initial order -- AA in 1935 -- had the passenger door on the RIGHT side, and used WRIGHT engines. When United ordered it, they called for a LEFT pax door and Pratt & Whitney engines. The aircraft was really successful prewar, orders totalled about 800 at the time of Pearl Harbor. This number of civil orders was not reached until the Boeing 727 in the 70s - maybe 35 years later.
Although the C-47 had the cargo door with a paratroop door-within-a-door, the C-53 had only the paratroop door - no cargo door. Both used the P&W engine, although in civil use some operators converted them to Wright engine to standardize their fleet. Purdue Aeronautics was one such example.
Generally, passenger conversions of the C-47 had the cargo door riveted shut, and only the smaller pax door was used.
There were several mandatory mods as the result of service experience, the wing attachment flanges being one example.