A line converter is the best way to get the signal to the amp. Yes, sound quality will drop a little, but it does not sound like you are going to any sound competitions with it. RMS out of the converter is not an issue as you are just looking for a signal to amplify (with your new amp). Infinity makes a sub kit that is plug and play as long as you do not have the H/K sound system. It can be found at bavauto.com.
They did not offer a 318 in 2000 in the US market, but is was available worldwide.
"It's not because I'm an aggressive driver, it's because I'm an idiot" actual quote from my wife.