My understanding is that such payments were SOP for Trump well before he ran for office. So showing that it was a political rather than personal family-preservation move would be very difficult—and take him out of any legal jeopardy.
Of course, this is all political now, and the worry is whether if a Pelosi House advances impeachment to the Senate enough Never Trumper GOP Senators would cross over to do him in.
Given their hesitancy, I'm not even certain the house could advance it to the Senate. Simple majority needed there. They would get their jollies simply to make the case in the house for all to see, without the need to advance to the Senate where it would fail.
Their goal is to damage Trump, and defeat him in 2020, knowing removal from office has never been successful, either failing in the senate, or forcing a resignation.