Its much easier to get partners for an existing plane than for one you
are thinking of buying. People just don't seem to want the hassle of
selecting and getting the plane.
The partners need to have ownership in the plane in order for the
insurance to work. Otherwise you'd be renting the plane and paying 3X
for insurance.

Personally, I wouldn't partner with someone who couldn't afford a plane
on their own. You just never know what could come up. I've partnered
because a plane turns to crap if it doesn't get 150ish hours per year.
However, I may need to write a $10,000 or more check at any moment if
something happens. A Mooney owner near here just paid $40,000 to
replace a rusting rear spar. That kind of thing can pop up at any time.
-robert