Don't worry about the type of steel. If you get a lugged frame or a Schwinn Varsity or Continental, a lower end frame is going to be plenty strong enough. It will just be a bit heavier. It will ride a bit more rigid too, but you can only tell if you run the narrowest tires at high psi

Steel rims are fine provided you don't ride in the rain or snow much. You will have to run a gumwall tire at a lower pressure because they won't have a hook bead, though.

You can get great, flatproof, skinwall, gumwall, modern, vintage, cyclocross knobby, road, touring, cheap, or high end 27" tires.

Looseness is something to look out for in the hubs, pedals, and steering. It may mean you need to replace parts, depending on how long the previous owner let it go. See for example if you can wiggle the rim from side to side in the frame.

Grease separates and the oil seeps out sometimes, depending on the type. See if there are dusty stains around the hubs, pedals, or steering. If so, drip some oil in there at a minimum, or get the parts repacked.

French bikes from before the mid 1980s come with unusual dimensions on certain parts, making them harder to replace.