Police:
Sell any vehicle but Police cars and the small helicopter. just spam police cars.
Either Protected gear or Shotgun SWAT gear.
Have a few cars where one crew has a bomb suit spread around the city.

Medical:
Spam Transport vans. Give at least one guy the Professional Life Saver.

Fire:
Spam Brush trucks and give everyone technical tools.

A good ratio would be 3:2:1. 3 police cars, 2 fat medical vans, 1 brush truck.
Police incidents are by far the most abundant.
Vehicles are more important than gear.
Police incident icons are important to note. If there is a set of handcuffs, then this incident will require at least 1 empty spot in the police car. Police badge icons won't require empty seats in the police car.
Fire incidents icons have either a wrench or a flame. Flame icon will test your firefighters "fire" skill AND SOMETIMES Technical skill. Wrench incidents test technical skill.

Note the path finding. Some cities have weird highways setup where you have to loop around to get inside a certain area.

As your force gets bigger, OVERTIME becomes one of your biggest costs. Don't hesitate to mass ignore incidents when the timer runs out. Otherwise you'll be paying 100 vehicles and staff while waiting on a handful of staff to complete the last 10 incidents.