Preston no doubt its overkill for many projects but the pot adjustable sensitivity systems are not actually user friendly. For variable speed assist it is far better to have a simple map-able ecu control unit with assist based on wheel sensor speed input. Of course this option is not a run down the scrap yard and bolt it on solution.Any standalone system that works from a drivers perspective is bound to be more expensive than a system that just makes the steering light.
Here is a link to a Corsa Opel (GM owned) eps swap that may be of some interest.
http://www.super7thheaven.co.uk/blog...steering-epas/