The minimum navigational equipment for IFR operations in most areas is 2 VOR + 1 ADF or 1 VOR + 2 ADF, ILS, DME, Transponder, Marker and 2 720 channel VHF Comms (below FL100 in Amsterdam you can get away with 1 VOR and 1 ADF, but watch out for Germany).
Don't forget the reason for Special VFR - it's to allow you to fly visually when the wx is officially IMC - if there weren't a requirement for rules, you wouldn't need such an exemption.
cheers
Phil