For most applications you don't need more than 300 ppi. If you don't intend to print your map, you can pretty much ignore the ppi and just work with pixel. But of course that's totally up to you. You can change the ppi-setting at any time and without any loss of quality: menu Image -> Image Size. When you do that, make sure, that "Resample Image" is unchecked.

8 bit color depth is enough for almost everything. Many programs can't even open or display images with 16 bit or higher.