I have seen models with plug wires that are too large for 1:25 scale more often than not, especially colors other than black. The use of the black #6001 was on an historic model and black wires for the 1930's were historically correct.
The 3ft wire is more than adequate for one or more vehicles. Importantly, the wire is a single wire insulated and not a stranded wire. Stranded wires are more difficult to shape and maintain their position. On the other hand, straightening the solid wire after it has been bent creates a challenge; however, I personally prefer the solid wire over stranded, especially in the smaller sizes. Overall, I am pleased with the price and usability of the 1:24/1:25 scale plug wires/ignition wires.
For fuel and brake lines I use 24 gauge nichrome un-insulated wire that I also use for building different configurations of Styrofoam cutters. Nichrome wire is relatively inexpensive and can be found online as vape supplies for electronic cigarettes. The wire tolerates repeated high heating without losing its strength of flexibility. Numerous designs for do-it-yourself Styrofoam cutters can be found on YouTube and they are relatively inexpensive to build.