If you are doing lots of design stuff, I would recommend you to
add. Because now I'm facing lots of memory drainage from just
editing one picture. It killed my laptop literally. 
If it's just normal stuff, like using Word, Excel and checking
emails, then it will suffice. If you are doing programming, it's
all up to you whether to add more RAM, especially if you are doing
Java. Java is nothing but slow, due to their platformless
nature.
As for batteries, if you don't mind the extra weight, you can have
it. Screen protector wise, it's optional also. If you want your
screen to last, get one. It can protect your eyesight too, though I
find it a bit too dim which seems to make eyesight worse...
