From the description, the rust sounds very minor. Rusty screws can be replaced very easily (usually) or treated with an anti-oxidant. Evaporust is a great product and is inexpensive. So is apple cider vinegar.
Financially, Featherweights are almost always worth putting in the time/effort/materials to restore. They command high prices and are considered the non plus ultra by many sewers.
If a machine runs well despite a little surface rust, it's not an issue and I wouldn't let it deter you. (Sewing on the machine is a good idea, especially if you're planning to spend over $100 on any machine.) If the rust is more substantial and indicates the machine was stored in a moist and/or unheated environment, I'd be wary. No matter what it is, Featherweight or no, another will come your way that's in better condition unless it's exceedingly rare.