I probably won’t get the sill repaired during my ownership unless the rust spreads. What do you recommend I use to remove the rust (if I can) and what then to apply?

 

That kind of rust is already spreading under the paint. It needs to be gotten off asap. It appears to be layered, which means you'll need to get it off with a sander, media blaster (you can make a DIY one for almost nothing) or wire brush then apply a rust inhibiting primer. The primers are readily available at hardware stores or the internet. If it isn't too bad, you may be able to find a body shop that could do a quick repair and save a lot of paint. All you have to do is ask for an estimate. You may be able to find a rust free color match fender on the interweb and it may not cost as much as the body shop and your time to replace it.

For the DYI cheap media blaster...