The other issue is how you want the Start button to operate, one touch with auto start sequence or starter operating for as long as you press the button much like early cars. My Roadster for example has an automated start sequence controlled by the powertrain control module. If you don't choose the Start button option a link is added to compensate but you still get the automated start requiring you only operate key in start position momentarily.
Upshot is if your car had a factory start button option when new it should be easy to retro-fit. Otherwise the only practical option is to add an old style starter button and wire in parallel with the key start.
