I would leave it until the summer now. in this weather your risking trapping moisture anywhere you try and seal it.
get the sill trims off. clean everything you can, wire brush what you can. kurust it all as best you can and then waxoyl the fuck out of it. I need to check my runabout starlet for rust in the sills. been on the to do list for a while!
it wont get bad that quickly.
kurust is a chemical that is meant to convert rust into another compound. its not a cure for rust. but reduces it and slows it down.
any larger patches of rust want to be taken back to bare metal or cut out if possible. but I don't think yours is too bad tbh.
I used some stuff from Bilt Hamber, this one to be precise:
http://www.bilthamber.com/corrosion-treatments/deox-gel
I had some rust behind the rear bumper, this stuff took it right back to bare metal and just needs a covering with some zinc primer or similar.