Dual mass flywheels failiures, a costly and common fix.

Inlet manifold runner control actuator can break, which also breaks the control box at the same time.

Its rare but not unheard of for the flaps inside the inlet manifold to break.

They seem to munch lambda sensors.

Rust under door mirrors, along bottom of doors/boot, around boot handle.

Personally if I was having another, I'd have one without climate control, its a god awful system on the mk1 focus.

Cam changeover solenoid thing leaks oil into spark plug wells.

Thats all I can think of off the top of my head!