If it were me, this is how I would handle those two questions:
Rust. Don’t worry about it. Rust is a patina, it won’t do any serious damage unless it gets too much moisture for too long and starts really deteriorating… Likely not to happen unless you bury it in the ground or something and not touch it for years and years. Both of my anvils get rust on them from time to time, just from moisture and humid days where the dew point is really high - so do all my tools in that type of condition. Don’t fret about it, if it gets really bad you can just take a knotted brush wheel to the anvil to clean it up, but generally rust is not a worry.
I wouldn’t worry about the heat treat and temper, looks like a railroad anvil that had a top plate welded on it? Do you know what type of steel that top plate is? The railroad will be tough enough and you won’t be harming that any time soon. The top plate, I’d say as long as it’s some sort of tough tool steel, you’re fine there too.
That is a great anvil for what you need it for Also, if you ever wanted to throw a hardie hole on there, a lot of people have done it with railroad anvils before. If you’ve got some welding experience it can be pretty easily done.
The only problem one may have is the mass of the anvil isn’t quite there, if I were you I’d get a nice stump to put that on, attach it real well to the stump maybe with some big nails or something, that’ll help add to the mass.