ABS problem can be many things. But a easy check is to check the first connector from the wheel speed sensor. If there is dirty, debris or corrosion then clean it out. Compressed air works really well along with a brass tube cleaner. Also depending on what channel ABS you have that will determine if you have one on the rear axle or not. You can easily check each sensor with a Digital/Analog Oscilloscope.

4 channel ABS has 1 wheel speed sensor on each corner.
3 channel has 1 on each wheel up front then 1 on the rear axle.

Your idle issue may also be a easy fix. As it seems to never really have a constant idle when you start up. It either high or low. You could simply have a very dirty throttle body which is causing it to stick. Once you apply the pedal a few times to get the throttle moving it may be moving the funk around enough to give it the movement it requires. Try simply cleaning it and see where that leads you.

Ok the key issue could be a bad sensor that is not detecting the transmission in PARK. Actually Im quite sure that is it. Im not sure what else it could be.