Physically the (front) seats fit from an M3 coupe fit fine in an E46 sedan. M3 convertible seats also fit, but you would then be left with two seat belts because the vert seats have the seat belts built in unlike the sedan/coupe which have them on the b-pillar. If you have power and heat in your seats, and you install seats with power/heat, then it will be completely plug/play. If you don't have power/heat currently, and the seats you're installing do have those options, then you will have to do some wiring to get those options to work (not to mention add the buttons for the heat settings in the centre console). If on the other hand you have the power/heat settings, but the seats you're installing don't have them, then it's still plug/play, but I would remove the fuses for those options simply for the sake of safety.
I hope I've explained it clearly.