There's a few things one could reasonably infer to be part of the basic ECM package.
>basic radar jamming and deception suite(but not chaff)
>standard and IR dazzler of some sort
>simple heat decoys
>maybe smoke for IR and visual
Anything more than that would reasonably require the more advanced ECM packages.
The existence of light polarization, chaff pods and perfect masking from electronic detection as systems separate from the basic ECM package means those are abilities the basic ECM package does NOT possess.
Whatever particular ECM systems mech's have, it's enough to defeat basic mech sensors some of the time and common guided weapons quite often.