As mentioned, the stock wiring is 16 AWG (mids and tweets) and 14 AWG (woofers). This is PLENTY. On top of that, it's all twisted pair wiring of seemingly excellent quality. The E36 chassis is electrically noisey, and twisted pair wiring is designed to minimize induced noise. You'll have to spend a lot of money to get an equivalent quality wire with the same amount of noise suppression designed in.
As for the wires being old, that's not a concern- copper doesn't deteriorate signifcantly over time, and even if you do get some oxidation, cupric oxide acts as a barrier to further corrosion (think of the green copper roofs you see on some older buildings- unlike iron or tin which will continue to rust at an increasing rate as more rust is formed).
Finally, don't believe the hype. Companies spend big marketing money to convince people that their proprietary oxygen free copper and tungsten-welded blah blah is 800% better than the competition. Time after time, independent blind listening test have shown, even in anechoic chambers, that the audible differences in cabling are extremely miniscule, or even undetectable by most ears. Factor in the mobile audio environment, and the difference is even less.
SOLD- Now Enjoying Honda Civic