I would really recommend NOT using a belt sander, unless you're VERY confident of your skills, and can keep flat surfaces ... flat.
If you put a slight convexity into a sole or (worse) a frog base, it will cause the parts not to fit together securely leading to chatter, and general planing misery.
Removing rust is (I find) pretty easy by simple wet 'n' dry. One of the "secrets" is to start with surprisingly fine grit. 240 is about the coarsest I use, and that not very often. 320 is my normal starti8ng grit.
Rust is fairly easy to get off - but scratches left in iron and steel by too coarse grit is much more difficult!
BugBear