I don't know that the Comey memo would provide evidence for an impeachable offense. I'm not sure merely asking him to drop an investigation rises to the level of obstruction. It's bad form, but probably not obstruction.
But, honestly, who doesn't believe that Trump asked Comey to let Flynn go? Of course he did. And, Trump himself admitted to firing Comey over the Russia investigation. That's a bigger scandal to me than the Flynn memo.
I think the target of all these investigations is Flynn anyway. He's a shady character
Regardless, impeachment would require about 20-25 Republicans voting to bring charges. That seems unlikely. Trump is clearly incompetent and unqualified to be POTUS, but I'm not sure he's done anything that rises to the level of impeachable.