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As the subject says - or do you have to a buy a knight kit with it in?

To the best of my knowledge, you must buy the Knight Preceptor/Canis Rex, which to be honest is the only knight you should buy unless the £10 difference, at most on the GW site directly, is too much for you.

This may be less for your benefit, and more for anyone else interested in which kits have what, but still:
The basic Knight kit can do either an Errant or Paladin, with Reaper and thermal cannon and rapid-fire battle cannon in the box, with heavy stubbers.

The next kit builds Gallant, Warden and Crusader, along with the Errant and Paladin, and includes the sprue with the Gauntlet and Avenger Gatling, as well as the frame of carapace weapons with all 3, some more customising bitz like alt. face and tilting plates and the meltagun.
Finally, and crucially for the same price you can get the Preceptor kit, which has all the above, as well as the las impulsor, a multi-laser torso mount instead of the stubber and melta, a pilot to sit in the cockpit unit, and a pilot to have on foot as Sir Hektur if you build it as Canis.
So, in essence, for the same price as a Warden/Crusader/Gallant (itself at no worse than 15% more than a no carapace, stubber and reaper chainsword only Paladin/Errant box) you can make a Knight for any datasheet in the codex, and with all the weapon option available, with all the custom options you could want short of 3d printing armour plates with your own insignia, and with the pilot.
I have yet to get myself a knight, but I know what to get.