Whenever you keg a beer, you should always purge the headspace of atmospheric gasses by pressurizing it with co2 and relieving the pressure 5 or 6 times (well, I do it 6 times), and then bring it up to pressure again with co2.
Kegs seal from the inside, and will not seal properly without internal pressure.
7psi bare minimum. 12psi reasonable. 20psi not unreasonable - you were planning on carbonating it anyway - but if your co2 system doesn't have a check valve protecting your regulator, you should make sure that you release the pressure before hooking it up for serving.
And, a keg is a dark place