The stock wire is about 20 gauge tinned copper but you can make it a little thicker.
Most people are using the Delphi-Packard Weather Pack connectors because they are inexpensive and work well on outside of the e-box and the original Japanese terminals with the boots work on the inside of the box.
Also some people use the original white color Japanese style connectors or the MSD weathertight and Deutsch connectors but they are a bit more expensive.
All the waterproof electrical connectors are available at jet-world.net or on fleBay.