Preshading

I guess it depends on whether you want it to have a "weathered" look, or a factory fresh gloss white.  If the latter, I would put the white paint over white primer and forget about preshading.

If you do want some preshading, I think a light gray primer would work best.  But I would suggest doing a test run on a piece of sheet styrene first to see if you get what you want.

Yeah, whether you want it weathered or factory minty fresh matters. Also keep in mind that the vibrant orange and colorful markings will smash down any contrast among the white, so it's probably going to be a good call to overdo things!

It's going to sound counterintuitive, but (again...if you're going for weathered) I would paint it black (or very dark gray) first, and then white over that. I've tried various preshades for white in the past...found dark colors too stark against the primer, and light colors too easy to cover up. Going all black makes it easy to "subtract" shading very deliberately. 

Here's the process I used on a Swiss 109 I recently polished off. First, base coat in black:

Second, white. This is Gunze C69 Offwhite, which is the absolute best white I've ever used. It looks pretty splotchy here, but as more colors are added the contrast among the white will get crushed down so your eyes don't really see it.

And done. 

Thanks much for taking the time to reply. I've really had coverage problems with gloss white in the past. Starting with a black base worries me. I'll try on some scrap as you suggested. Thanks!