I'm about to build an X-wing and the same thing applies. I think just go with dirty off white. Prime with black and apply white mixed with a bit of tan.
Disclaimer: this is what I intend to do but I haven't tried it yet.
That sounds about right. I'm using a pre-shading technique to make it look more worn (a less subtle version that what I did on my G2 Grimlock). Giving it a worn look can hide a multitude of sins