Just for general usage (like watching movies, listening to music, MS word, etc.) - do you think 256GB will be enough?
I hear some people say that 256GB is too small for them, and I honestly don't see how that's so small. Are they like professional photoshop users or something?
Also, is the SSD on the late 2013 MBP with retina upgradable?
It depends. Everyone is different. Some people have a lot of music, others not.
I know people who are happy with 128, personally I'm not happy with less than a terabyte and a half.