Scissors come in a variety of sizes and prices. For paper engineering you'll probably need a couple of different pairs of scissors. A small pair, with short blades for doing intricate work and a pair with longer blades for scoring and for cutting out straight lines.
If possible, try your scissors before you buy them. Make sure they cut cleanly. Check the full blades for nicks or catches and make sure that they open and close freely. If they are stiff they will quicky become tiring to use.