If you include a 'slug' it will add color information etc. to the file. Sometimes, if you see an uncut printed page, you'll see colored squares at the top or bottom (outside of the print area that will remain after the page is trimmed).
To change the default measurement from px to in or whatever, you change this in your Preferences settings. (On a Mac, it's Indesign > Preferences > Units & Increments.)
When you are choosing colors, make sure they are CMYK and not RGB. In terms of ppi, the main thing is to make sure that any graphics you create in Ps are large enough that they will print well. You don't need to do anything else about ppi/dpi in InDesign.