Learn English – “would I need” or “will I need” What is the correct sentence

The copyeditor in me observes that your at there adds nothing to the sentence: leave it out.

It’s also something of a shibboleth whose use will mark you as “poorly educated”, because these things are expected to have been corrected in the course of proper schooling. The same is true with questions like “Where are you at?” Here again that at at the end adds nothing — or at least, nothing good — and should be omitted.

Once upon a time, surely aeons ago now, there was something of a hipster phrase about “where it’s at”, referring to some location or event or even style that was supposed to be “really happening”. Unless you’re trying to faithfully recreate the special slang from that period in history, I would avoid it.

You can say I don't know Mr. X personally... and there's nothing informal about it: it's just a statement of fact. You might want to explain a bit more about how you do know him, though: ...but I'm familiar with his work...