I would like to leave the Limber, once built, in its bare metal state without painting it. Right or wrong I think it will look better that way. I could always paint it later.
Since it's my first metal build, I have some newbie questions below.

I've read the answers to Ian Cameron's questions.
That means I believe that I would have to clean and degrease it:
I'll look for a French replacement for Upol System 20 greaser. That should be too much of a problem. There's a small auto repair shop across the street from me.
What do I use for polishing the piece to get rid of what appears to be some of the marks on the metal surfaces and near/next to the drill holes (oxidation perhaps) and what sort of tool do I need in order to do it?
Perhaps I should also coat the surfaces, as I build it with some product to prevent oxidation.
Any suggestions on how to proceed would be very appreciated, including some product names and/or sources. I live in France so generic names would be appreciated. I'm not sure what is available on this side of the channel, model engineering is more of a UK specialty then French. Most people that I've known have ordered things from the UK, I've had to do that with the rivet snaps.
Merci in advance for any suggestions and help.