There's a common belief that once steel or iron begins to rust, unless you remove all the rust, it will continue. This being said, I've known lots of people that have inherited rusty iron pans, given them a light sanding, seasoned them and used them for years.
Now, in this particular scenario, where the plate is this close to the broiler, I'm relatively certain that the surface temps will hit sufficient temperatures to cook off any seasoning. I'm not 100% certain that a little bit of embedded rust on a lightly sanded, unseasoned surface would be the end of the world, but, since I've never tried it, I'd probably go the extra mile and remove the rust.
If the rust wasn't deep, I'd sand by hand, using a sanding block. Maybe it's just me, but I feel like I can maintain a flatter surface using a sanding block than an orbital sander. As far as the scratches left by sanding go... I don't think they're a big deal. As long as you get the rust off, that's all that matters.
The evapo-rust just looks like an expensive mild acid. If the rust was moderately deep, I would probably just use Naval Jelly, rinsing it carefully and baking it quickly to hopefully prevent embrittlement, although, on a hunk of steel this size, I kind of doubt embrittlement would make any difference.
If the rust was especially deep and the surface was pitted, I might opt for something like Rust Removal using Electrolysis
One benefit of electrolysis is that it should tint the surface black, which would be good for heat absorption, although not completely necessary.