"It would be nice to have a system that worked well, had a cross bar near the windshield, and I did not need to start drilling the roof of my newly acquired car."

you could go "hybrid" and use the factory roof rack for the back and purt a Yakima rack up front wherever you want it - Q towers and Q-clips don't require any drilling and the clips are customized for a particular vehicle, so you could check availability before buying the car.  That's what I use on the cab roof of my dodge dakota - the clips are mostly held in place by the closed doors - unless you drive down the highway with your doors open, they will be solid - you'd need a cross bar also, of course.  That would be much better than foam blocks, though I have used them and if you put some sticky shelf liner under the block, they don't move around - but its a lot more fuss to tie the boat to the roof using blocks vs a rack.

I notice that a lot of cars nowadays have a very curved roof line - putting a cross bar way up front too far may not work if the roof slopes down too much - might not be enough clearance to keep the boat off the roof ?

one other point - many factory roof racks are more for looks than function and aren't at all strong - double check with the manufacturer to be sure they are strong and functional.  I have seen reports of factory roof racks flying off of cars and have seen one personally being very loose as the bolts worked loose on a Honda CRV - you could lift the front left corner up an inch or so before it got repaired.  Bow and stern tiedowns help hold any rack to the roof.