once you reach level 15 or 16 and have enough Cash you can go with 2 or 3 cds to have a better car.
here it is right that it takes 6 races, when the cd is on manager level, to get the max of 50 next season, with one limitation the strenght must be the strenght for 6 races and must not be the weakness for the other 11 races.
I don`t remember reading that on any of the guides, but then again it`s been awhile. That puts a definite solution to any frustration on CD management.
There are some mentions about this in the comments in one of the guides.