Sculpting crisp mechanical shapes can be fussy work no matter what clay you're using, that's why working with materials like styrene, wood, brass, or cardboard is usually preferable. If you use clay for that, you want something that's rigid and easily carveable when cured. You sculpt it roughly to shape you want, then after it's hardened, you carve, scrape, and sand/file it down to get the flat surfaces and crisp edges you need. This may involve several passes as you will likely have to go back after the initial carving/sanding to spackle in dips kinks, nicks, voids, etc. Don't worry if you have to do that numerous times, as that's pretty standard.
Of the stuff you listed, Milliput would be the best for this. I'd actually recommend Aves or MagicSculpt over Milliput, but you said in your other thread that you already have some Milliput on the way, so use that. You don't want to use Green Stuff or paper-based clays because they don't carve or sand well. Plus they're harder to sculpt into crisp edges to begin with, and the closer you can get while the clay is "wet" the better. Fimo would work okay, but Milliput is probably better (in terms of carving and scraping and such). Brown Stuff would also work well, but it's probably better to save that for figure work and fine details, as it's better for that than milliput, and use the milliput for the bulk shaping.