Upgrading to brushless is usually a good move, as it increases efficiency and simplifies maintenance.
As for the spec of the brushless system, it often helps to check if your car has a brushless version made by the manufacturer, and seeing what the spec is of the power system supplied with it. This has usually been tested to ensure that it is a good match to the chassis. You can therefore use this as a guideline for choosing the system to fit to your own car.
For example according to Modelsport the brushless version of the Maverick Strada XT comes with a 3215kV motor, so choosing a motor of the same or similar rating ought to work well in your car. The HPI website indicates that the stock ESC has a modest 45A rating, so anything over this ought to be plenty.