The good thing is that drywall knives are not expensive. My advice is that you purchase low-end knives because, if you are like me, you eventually will toss them out and replace them. Perhaps if I were more fastidious I would not have this problem. Yet no matter how hard I try to clean off my drywall tools, they eventually get gunked with impossible-to-remove drywall residue.
The 6 inch knife and the 4 inch knife are used for slopping the mud into place and for taping. The 12 inch knife is used for feathering and final coats. You will need either the 4 or 6 inch (they are fairly interchangeable) and the 12 inch knife.
Hyde Tools sells a nice, off-the-shelf 4 inch drywall knife, perfect for taping.