The O2 sensors on my 500SEC are single wire, but they may have been fitted when it was imported, when the cats were fitted.
They shouldn't be hard to fit, as said above, just splice in sensors with the same count of wires, making sure to connect the heater and sensor wires correctly. (12v constant to the 0-1v output can't do good things to a sensor!)
As far as the ticking goes, you may find that running good oil (possibly a good synthetic for a cycle?) will clean up the valve adjusters and reduce or eliminate the ticking. Mine ticks slightly, but only really audible at low rpm.
You also might consider getting a local shop to pattern check your injectors when you have them out. They're simple nozzles with a pressure 'relief' valve that holds the fuel in the line until the pump pushes it above the release pressure, but if they dribble below that point, or stream instead of misting, they cause interesting issues. The dribbling makes for long cranking to start when warm from what I've seen, whereas the streaming makes for rough running.
It's good to hear your toy is behaving better for you though!
Happy wrenching, and best of luck keeping the foot out of the fun pedal!
