Peter
In my opinion, it depends on your Manager Level.
In the early levels (Level 1 - Level 8), the 'Design' building is more important, it it allows for a stronger base car over the season, with lower level KERS having not much of an effect.
For higher levels (Level 15+), definitely the 'Technology' building. KERS duration in particular, becomes paramount in very competitive and active leagues, especially towards the final stints of races.
Hope that helps, and good luck! ;)
What you said seems to make sense. DRS won't be good if your car still runs slow. A good car may not need DRS if u have a good speed