I am going to pick up a police trade in P226 in a couple days. In case it needs it, what springs should I replace to have a easier time of it in double action?

I run this in mine.  18lb hammer spring.  Around 1K and no light strikes.

Wolff Hammer Springs

18# is what I run too

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.

good thinking

The only thing I would add is to replace all of the springs that you can. They are cheap and are easy enough to replace. When I buy a trade in,that is what I do.