The smallest most robust medic I could think of that still looked reasonable to me. The wheels are wide mounted (3 block width) to keep it stable at speed and the single forward mount gun is able to repair the full circle around.
To make an impact in rank 3 battles it needs to be driven across the battle field to repair at each of the separate fights occurring. Staying with one group doesn't work as you'll often be the only medic and need to repair everywhere at once.
This is a good practice vehicle to learn how to keep your vehicle under cover while repairing from maximum range. Literally at this level if you go in close you will be torn up. Aim healing is useful as you pick up the team member playing well rather than the one being dumb.