Frank, depending on the application, you would need 10-12 gauge for the high amp stuff and 14-16 for the medium amp stuff and 16-18 gauge for the low amp stuff. The important thing is to use relays where required and fuse everything with the correct amp fuse.
If you under dash wiring is shot, the rest of the car’s wiring may need attention too. Why not in think of using a complete Painless Wiring (or other brand) complete harness. All the wires are clearly marked and color coded. You can buy as many circuits as you require and add to them at any time too. I think you’d be happy with the result and the investment.
There are also many good publications avalable for wiring cars. One is called Hot Rod Wiring and it’s very useful for kit car applications.