Parts break... it's a fact of airsoft life. In my experience, Modify makes the best all-around parts. Systema is very good but expensive. If you're worried about the life of your mechbox, don't upgrade the FPS too much and get a sorbo pad for your cylinder head.
For the highest ROF:
1) Keep the spring relatively light. A lighter spring is easier for the motor to pull.
2) High speed gears and a high speed motor.
3) Lipo batteries (even without the above upgrades, Lipos can turn your gun into a beast).