The Knock-Down action is an opposed Strength test. It says nothing about needing to actually hit whoever you are trying to Knock down first.

Yet I think the targets Agility and ability to evade should enter the picture here, otherwise some big, slow brute will routinely swipe the floor with the agile assassin. Knock-Down, Attack. Then the Assassin needs to Stand (Half Action) and might have taken damage and fatigue. This severely reduces his chances. Rinse, lather, and repeat.

Have I missed something? Can you Dodge Knock-Down? At least Grapple requires a WS test to Hit, that can be dodged.

I'd say you get to resist with dodge but if you try and fail you get -20 on your opposed strenght test. The reason being that if you try to dodge and fail you are going to be off balance and much easier to knock over.

I find that knock down looses a lot of value if the target gets to try avoiding it twice (dodge and STR-test)

Personally I let the Defending character choose between STR and AGI to "resist" a knock-down. Meat-mountains will try to "tough it out" and the nimble assassin will dodge out of harms way.

You only get one Dodge a round. I'd say you'd be allowed to use it to avoid a Knock-Down attack.