Pencil: If you sharpen it regularly with a pencil sharpener, an ordinary pencil will do.
Rule: An 18"(500mm)-long stainless steel rule is sufficient. The metric rule must have 0.5mm marks and the imperial rule must have 1/64" marks over the entire length. If you want a high-quality rule, get a Starrett one.
Square: Buy a quality steel square.
Caliper: A quality caliper should be in every guitarbuilder's toolbox.
Thickness gauge: I use a Japanese-style thickness gauge. The excellent Veritas gauge is shown on the right.