Wire. Flexible type (high strand count) in several sizes (10 to 18 at least)
Wire strippers. The 'kinematic' type (like
these ).
Contacts and housings (depends on connections you use. Recommend Anderson Powerpoles)
Screwdrivers (very small blade-type to mid-size philips)
Pliers (including needle-nose)
Wire cutters
A soldering iron (25 W) and solder
Electrical tape of good quality (black)
Heat-shrink tubing assortment
Crimp-type insulated spade, ring and 1/4" terminals in various sizes, and a RATTCHETING crimper for them.
Wire ties: 1/8" x 4-5", 3/16" x 12", 1/4" x 12". Hundreds of them.
Some spiral-type wire covering
Marking tape: colored tape or label-maker tape to mark ALL wires at both ends.
That's what i can think of at the moment