Welcome.
#12 wire will be fine for a stock CB. If you're running moderate power, use #10 wire. You should be able to find automotive grade stranded #12 red wire at most auto parts stores or via Amazon.
If you are lucky, there will be a knock out gromet on the firewall. If not, there may be room to coax the power wire (maybe with lube) along the path of the main wiring loom.
Marine grade spade crimp connectors work well. These connectors have a heat shrink sleeve with adhesive inside that seal the connection very tightly. If you don't have a crimp tool, solder on crimp terminals will work along with marine grade heat shrink tubing. Add a fuse connector close to the battery connection. You can use the vehicle chassis as the negative connection, but make sure it's a good ground return.
Wire "insulation" does nothing to mitigate engine noise (static??) and there's no easy solution. There have been some good discussions here on RR on various techniques to mitigate static that you should be able to locate with a simple search. Part of your "noise" issue may be the magnetic antenna mount.
