Treadling isn't that much of a workout. I just sort of bounce my right ankle up and down, without moving the front part of that foot much at all. The left foot is just there as a sort of counter balance, or to help it stop. Not enough movement to really count as cardiovascular! It's about as much work as sitting and tapping your foot as you wait... think of it that way.
The machines probably won't be hard to recondition as long as they aren't heavily rusted inside. Liquid Wrench works wonders at getting things freed up again. Then you wipe it off an apply new oil (maybe grease in a couple of places) and you are good to go!
Just oil as needed, after that. You usually know when you need oil, as a machine will get a bit noisier than it usually is.
Treadling is so peaceful and fun. Don't know what it is, but I always want to go to the treadle first...