You may need to get 12mm spacers for any wheel under 18". For everything to work, you'll need new calipers (obviously), rotors, pads and brake carriers to hold the new calipers. You may want to throw in some reinforced brake lines if you plan on some tough braking.
<span style='font-size:8pt;line-height:100%'><span style='color:blue'>1998 BMW 323is Coupe, Black
18" M3 Hyper Silver Rims
2 12" Rockford Fosgate Subs; Sealed
1.0 Farad Capacitor
800 Watt 2-Way Rockford Fosgate Amplifier</span></span>