First how soon do you expect your 6 year old to start racing??? If it's within the next 6 months to a year then worry about raceablity, if any longer than that I wouldn't care at all, cause if he uses it with any regularity then he'll have to worn out withing 2 years, if he doesn't then getting him to race maybe a lost cause.......
Personally I suggest a Tamiya vehicle cause they will outlast most of the carnage that he can dish out.... Also the Tamiya XB cars are upgradeable, just not to the extent that the Associated cars are....