You just need to purchase one sized for the motor if you already have 3 phase. If you are using a single phase source to run a 3 phase motor, the rule of thumb (contact specific manufacturer on sizing specs) is to go double size (10 HP vfd for a 5hp motor).
Power wiring is pretty basic for incoming and outgoing power (except in single phase input to 3 phase output, specific terminals must be used typically).
If the intent is to use 3 predetermined speeds, you would most likely need to wire 4 inputs but will have to read manual for specific drive. Most would require an input for each speed, and an input for 'run'. Wiring would be set up where the car wash 24vac wiring would operate signal relays, and those relays would throw voltage from the vfd controls itself (likely 24vdc), back to the vfd input board.
Then it's just a matter of finding every correct parameter required to configure it. Usually there's 15 or so just for setting the motor up (newer drives have an auto tune feature where the drive configures the motor parameters itself by 'sensing' the motor wiring). Likely would have to set parameters for remote run by input control, run using preset speeds, run signal from input wiring, coast to stop, speed for each input, ramp up speed per input, and others for anomalies like what to do during phase loss, alarm conditions, and fault conditions. All of those settings have default parameters, but are basically set up for running the drive only using the front of the drive buttons to run a motor at 60 hz.
