Suggestions on how to control a 1000W DC motor
Should I use 100A-rated contactors, for example?
  
  May be, relays are too slow..


Depends on what you want to use them for!

Relays would be sort of "ghetto-rigging" it, but if thats fine for the application then go for it.
If you're going to be switching them on and off a lot (with PWM or something) a solid-state relay would be worth the extra money.

That being said, just get an off the shelf motor controller for 1000W DC brushed motors. It will make your life much simpler.