Which version you go for I think is something
you should base more on what you're comfortable with and what you're using in the marketplace. If you're just now starting to prepare, I'd suggest going for 5.0, if only because it's the latest version.

I don't know that any employer (save for one whose interviewers have taken the exams themselvces) would know or care if 5.0 is harder or easier than the 1.4 exam.
And to answer your other question, no -
SCJA is not a prerequisite for the SCJP exam.