Go ahead and do your work in RGB at 8-bit color depth and 300 dpi resolution. When finished, duplicate the image into a new file and convert the new file to the CMYK color mode and send that to the printer.RGB has a wider range of colors than CMYK so you might see a shift in some colors. When you use the color picker if you use a color that isn't in the current color gamut (printable range) you'll see a small color box with an exclamation point above it. That is the CMYK color closest to the one you are pointing at. Click on it to select. If you don't select it, the color you pick will show up in the color box in the tool bar and color palettes, but PS will paint with the in-gamut color.I'd check with the printer and see if it would be wiser to stick with a Pantone or other approved print color system. That way you'd know what the colors will print like and you'd automatically be able to pick suitable colors to work with.Make sure to get a printed proof of everything before signing off on the full print run.