Personnally is not a good ideas to delete the original files or the working files I prefer to delete the disk image of bigger projets because the disk image you have it on your DVD.
Buy and exterior hard drive with USB and backup at least the original video captured, working MVP files and pictures. after you can clean at least the bigger gig files.
USB backup hard disk can cost $69 for a 500 gig capacity or $129 or less for 1T 1000 gig capacity and savea lot of problem just connect like a USB memory and copy on manually or use the software coming with the unit some free.