Rust repair and coverage on High temperature fixture
First, that looks like a common Cadet heater that costs about $50, so don't sink $200 worth of labor-of-love treating it.  

Second, it's high-temp, but not that high-temp - it's designed to have a relatively huge heating element with low heat per inch, so you won't burn yourself on it if you touch it. These things are in homes with children after all.  

So ordinary rattle-can spray paint should work fine, you don't need to use grill or stove or brake caliper paint.  (in case you want a bright red heater lol).  An alternate option if you have a Maker Space available to you with a big powdercoat curing oven is, powder coat.  That is how it was painted from the factory.  Since part of powdercoating is curing it in a 475F oven, obviously the coating doesn't have a problem with heat.  

If you really, really want to brush and roller it, you can't use a "latex" paint because the heat will damage it.  You need to use an alkyd paint.  The messy ones involving thinner and NOT being able to clean brushes in the sink. (This is exactly what is in "rattle can" spray paint, but obviously that doesn't bother you there because you don't thin it or use brushes).  They now make waterborne alkyds which are supposedly best of both worlds, but I doubt their durability.  We shall see.