2 movies would take about 2GB
300 songs about 1GB
20 books...like 1GB? books don't take that much space
Now the 100 apps that depends what types of apps they are. Games are considered apps, some games can take up 500MB each. But of you are referring to say small apps that take up each 10MB, that would mean 100 10mb apps would take up 1gb.
So your total amount of space you would take up would equal 5 GB at least.
If you get the 16 gb iPad you would have 11 spare GB's, so 11 GB's to waste on apps if your apps were larger than 10mb each.
You'll be fine with 16gb, but if you had money for 32 then Go for it