Are you going to use all the features that comes with that $50 keyboard? Don't you have a spare one on your house that work just fine? Think about it you could save some money from that and aim for a better CPU or GPU, its up to you. You could squeeze a little more money doing this for better and more important parts/hardware like CPU or GPU, IMO the keyboard and mouse really are "luxuries" when building a rig from scratch you need to focus on the more important things.
PS: Take this like a grain of salt, this is my advice, some may differ but this is what I believe
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