Another question... I intend to have the mount where the roof will be a ground plane, but it's possible that might not happen. If I run a NGP antenna on a ground plane, will that cause problems?
Will not cause a problem.
A 1/2 wave "no ground plane" antenna will have 0dB of gain without a ground plane under it. It should tune down to low SWR without any issues.
A 1/2 wave "no ground plane" -with- a proper ground plane under it should give you about 2.4dB of gain and tune up just fine.
If you are going to do a permanent mount NMO for these antennas, make sure you consider suitable ground plane size for what you are doing:
GMRS/70 Centimeters will want 6" of ground plane in all directions under the antenna.
2 meter band will want around 19" of ground plane in all directions under the antenna. That's why center of the roof is ideal.
I know a GP antenna without a ground plane won't work out well at all. I want to make sure I have something when it gets installed.
The NMO mounts have some 'teeth' that bite into the underside of the sheet metal. As long as you install it on some sort of conductive material, and tighten it down reasonably tight (knuckles should NOT turn white for this procedure), you'll be just fine. You do not need to sand/grind the metal under the mount, the teeth will bite in just fine. You absolutely do not need to sand/grind around the top of the sheet metal under the antenna.
Long term, you'll be very happy you put the effort into a permanent mount. It does help performance and looks way more professional that the compromise mounts.