Comparing versions of programs using REXX is not recommended. Programs tend to have a large number or records and REXX is generally not appropriate for processing such large datasets.
Also, when comparing records, you will discover not only 'missing' items but also mismatches which would also need to be handled - you can;t just blindly accept differences. There are several products already available to assist in this type of work
You really do need to start with something simple and work your way up to something as complicated as what you are proposing. Start with Hello World and progress to reading a file then copying a file then editing a file etc. Then add ISPF like a menu panel to select which of the previous programs to execute.