RD-407 is what I used, and it worked out fine. Not sure why some say no, seems like that's what most people use. As far as brand, I used monster makers, but there's plenty of suppliers out there that offer the same stuff. 1 gallon will be enough with a little left over for other small projects. I also recommend getting something to tint it with while your at it. Tinted latex will help cut the color of the suit, meaning less painting of the finished latex. Hope this helps.
Brian