Get the night sights. They make it easy to "orient" yourself to the gun in the dark after being awakened by the back door being kicked in (you do NOT need to be fumbling with trying to find a gun at such a time!). They also make hitting easier in dim or rapidly changing light situations such as near a street or highway at night. I have a friend in Miami who was attacked by two car-jackers as he exited his vehicle to unlock his security gate one night...he was very happy that his Sig 220 had tritium sights! They are NOT a "gimmick".
You certainly do need to ID your target, but I think some SOB who kicks down my door at 3AM has pretty much announced to me and the world that he is a bad guy! Still, you should have a good source of "white light" as well as a gun. My preference is the SureFire C2, but there are others.
If you can get out the door with a NIB G21 with high caps and night sights, tax, tag and title, for $600 that is a very good deal at any shop. A used gun with 10 rounders shipped to an FFL who will charge you for the transfer may, or may not, be a great deal once you pay all the associated costs, pick up some 13 round mags and have the tritium sights installed for you...most likely not all that much cheaper than the $600 one NIB.
The G21 is a great gun...you will love it if it fits your hand well...it IS a bit large for some shooters, but I shoot mine very well, indeed.