Ah well it depends how i'm feeling really, but usually i try and tackle the most difficult areas first, so i probably would start with that side section, seeing as it wouldn't really matter how many segments the disk section/access hatch needs as that would be easy to make that fit to whatever segment count needed to attach it to the rest of the body, once that's down should be easy to model the rest of it from there, parts like that where you've got a transition are always gonna be a bit tricky.