This assumes, first of all, that you're concerned with 2M and 70 cm.
1. Ground plane should extend at least 1/4 wavelength from the antenna in all directions. For 2M, think of a pizza pan about 40" in diameter with the antenna in the middle. If it's a magnetic mount antenna, be sure it's a steel pizza pan.
. This is a good coax calculator. Plug in the type of coax and the other variables, and it will tell you how much loss you'll be looking at.
3. Might be a problem, might not. Easiest thing to do is experiment.