You can just connect the green/black and green/red wires off the green 16pin connector to retain the dome light.

It doesnt really matter which you do first.
But, if you get the headunit, you will either have to spring a few $$ for a Y-harness to power the factory radio OR deal without having keyless until you get the alarm.
If you get the alarm first, just connect the door unlock/lock to the power lock wires in the floor/door harness. This will bypass the factory keyless (whether you have it or not isnt a factor)