Just depends if you are a multiplayer guy or not? Killzone is only really popular for it's fun multiplayer . l'd say it's worth the price for the multiplayer alone tbh. Unfortunately the campign is garbage so it really comes down to if you're fan of multiplayer. If you are l'd say it's a must.
l would say you wouldn't need to play Killzone 2 and 3 in preparation for Shadow Fall since the Killzone series is popular for its multiplayer. There is still a small community who both 2 and 3 online if you want to play multiplayer. The campaigns of both titles are good but obviously they are a little generic and the story events which takes place in both games will be irrelevant to Shadow Fall as it is a separate story.
But if you have time to kill l would recommend playing Killzone 2 and 3 since they are beautifully polished shooters.