Are You using the ADS-tracks for seperation?
Yes.
What is the required seperation in NM's?
I'm going to have to look that up, as the sectors I work on are radar sectors and we don't use ADS. I do know that by "one shotting" the ADS track (that's getting a current position) you can determine, using ADS, if the aircraft is outside a lateral separation point. I did use this when I last worked on a procedural sector with large amounts of oceanic and trans-continental airspace.
ADS is reportedly more accurate than radar as it gives the actual aircraft position as reported by the aircraft's GPS/RNAV positioning system. Radar has of course slant range and other inaccuracies.
As to the expense of ADS, I'm just a controller. Ask the bean counters at the airlines that question.