Thanks for the correction vilters... didn't have a chance to fire up photoshop when I made my post so my suggestions were coming from a somewhat foggy place. The actual values are dependant on the color space you are working in. 127 or 128 sounds about right. Looks like I've been using 102 from one of my reference swatches with no noticeable difference to displacement results... Shrugs.

I agree with obm890. I personally would create a subdivided proxy for the doors and create the deformations within a modeler (as OP has indicated that the obj is low poly, probably several large polys). Or use the putty tool. I doubt that subdivision would cause much in the way of distortion in the original uvs, being such as they are it's probably planer or projection normal mapping. Unfortunately OP has already stated that he has little modeling experience (or so I gather). Hence...