A lighter main spring will lighten pull a bit, but don't expect it to be like a professional action job.
How low to go depends on what you will use the gun for. Range only... I've run 17 pounders which reliably touched off everything except an occasional S&B which are know for hard primers. 18 pounders seemed to work for just about anything. For critical defense duty... 19# is as light as I'd go.
Police trade ins are usually US manufactured pistols with the newer style short spring and strut and plastic base pad. But check before you order a spring. You may need the old style longer spring, which BTW is a royal PITA to change. I would change out the entire strut set up if it had the old style spring deal. A few more bucks but it sure makes switching out springs much easier.