The M-Sport Suspension II as used on the 2003 540 M-Sport used M5 shocks and struts. That is the reason for the huge cost difference. Up to you if you wish to keep it original or not. But much of the mystic/allure of the 2003 540 M-Sport was the many special features and those shocks/struts where one of them. I verified the part numbers when I removed mine to install coilovers. Part numbers came up as the M5 parts
540 M-Sport package (Canadian market), factory Bluetooth phone integration, factory M-Audio 10" dual voice coil subs, Homelink, Eibach front and rear anti-roll bars, M5 rear traction rods, H&R 15mm wheel spacers, ST X (KW) coilovers, Strong Strut front upper stress bar, BMP strut tower reinforcement kit, Stop Tech ss brake lines, Akebono Euro Ceramic brake pads, Zimmerman rotors, Dinan Stage 1 software, USA Spec Ipod/Iphone integration.