I am telling you that I DO NOT want to say anything like it (under such circumstances I would say "This/That is the kind of potato I need."). I just want to know if it is something one COULD say or not. It's as simple as that.Welcome to the English forum, RafaelX.
I am puzzled by what you want to say.
Say you know that a friend has a potato and a carrot and a parsnip, and you know he has them.
You tell him that you are going to make a soup and he offers you a parsnip. You might very well say to him: 'I need a potato; the potato is what I need'.
A potato initially, because your need is for a potato, not for a specific potato.
The potato in the second case, because you've been offered the parsnip and you know he has a potato, so you say you want the potato.