Buy a combination set, they vary from just a few pounds upwards, for general work where it might get knocked about a cheap one will do, for precision work spend a bit more,
A Line of chords rule, (also called a 2ft platers rule in some industries) you can use it as a 2ft rule, protractor, angle Finder and circumference rule.
I should generally go for the combi set but honestly it depends on what your doing in the way of cutting steel. if its cutting box the protractor in the set is ideal for marking out. Not so good for the middle of a sheet
best to calibrate any cheap stuff before use