Chief designer

What's the best thing to look at when looking to buy a cheif designer?

What strength should I be looking for as well as a weaknesses?

1. 4.5/5 stars CD 
    P/s: Make sure your CD is at the same level as you.
2. Strength: Big 4 (Acceleration, Braking, Handling, Downforce)
    Weakness: Reliability and Cooling

Also with the new update getting a cd with fuel has strength gives a nice CD to use 1 or 2 

Oh I didn't know about that. 

Yes in a 21 races season, using one fuel CD during one race can improve your car development for the first races of the season, I bet it works in shorter seasons too, but in 22 races season it works perfectly!