All my bikes have their own GPS mounts, bottle cages and pumps permanently attached. I've only got the one saddlebag (topeak aero) which has a quick release system, so I can swap it over on to either bike quickly. That has 2xCO2, 2xTubes, Multitool, cable ties, patches, tyre boot, etc etc etc, I swap that over to the bike I'm riding. I was tempted to get a second saddlebag, but to be honest I ride my commuter most of the time, so it stays on that until I use another bike, when I swap it over.
Also, for your old bike, get mudguards, you'll thank yourself for it.
Oh, and what bike are you getting?