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I see. So for some laptop/PC gaming on chrome (no 4k needed) then it'll be worth looking at certainly. But to game on your 4K TV you need a sub plus buying games of choice, plus the initial purchase.
There is no initial purchase. For the 4k service it is just the £8.99 p/m fee. The pre-order package is totally optional.
You can use your existing controller/keyboard&mouse and screen of your choice if is part of a system connected to the net. You do not need a chromecast if you can run a cable from your internet device to your screen (as I do).