which would be best if buying a 4 post car lift with 3 phase motor, buy a converter to turn domestic 230V into 3 phase or change the motor to a 230V single phase .

Economics may also be a factor here, ie, what are the relative costs of an inverter as against a single phase motor of the required power. Plus of course as ES said what about the size of the single phase vequivalent, will it be too big to be accommodated in the unit?

Chances are the motor is three pahse for a reason, i.e. its not possible to have a single phase motor of that size with the same power output as a three phase motor, so you maybe forced to have an inverter.