The ground plane, as long as it is 1/4 wave or larger, won't degrade performance. I have a 1/4 wave antenna on top of my truck, and there's much more than 18 inches of ground plane underneath it.
As for antenna gain, as WRCM said, lower gain antennas can work in your favor. I'm in a valley also, and mostly in mountainous areas. 1/4 wave works well. Higher gain antennas can work against you in some applications.
I fixed a repeater system that had been installed by someone else a few months ago. The original installer had put up a 6dB gain antenna on a system that was on a very steep mountain side. The customer was having coverage issues. Putting up a zero dB gain antenna improved coverage by quite a bit.