Oh, the Jericho of course. I guess that's what I get for googling "spike spiegel gun" and not looking close enough at the random picture for the post.
I like to sandpaper at least the copyright info and Nerf logo off... you'll want coarse at first (low numbers) and then pretty fine for finishing (220 at least).
I don't have much advice on spray paints as I usually go cheap... just make sure it'll bond to plastic, be consistent with the matte/satin/gloss finishes you pick, and seal it with clear. One tip I will give is to put a bit o' tape over the screw holes, as even a little paint will (literally) gum up the works and make it harder to reassemble. You can always do some post-assembly weathering to cover up missed areas.