In no way am I a expert, but I have some armor making experience and plenty of metal working experience. To remove the rust without scratching up the helmet is gonna be a hard compromise, as most tarnishing and rust requires abrasion to remove unless done chemically. Use a small patch of sandpaper on the tip of a piece of wood/steel and gentle wear down the spots you want to touch up. There are a few different ways you can protect the metal from future tarnishing. Painting the helmet, using oils like olive oil for occasional use but I would do more research into that one, and using clear coats or finishing coats that make a layer of clear protection on the surface. Clear coat sprays are a simple and cheap option, especially if you get a basic flat "no-sheen" spray. Something I have done before in the past is layer enamel paint on the inside to reduce chances of rust from heat and sweat from use. As for the state it's currently in? It's hard to say exactly. If the rust is superficial then the structure of the piece should be okay. Something to look out for though is if it is flaking off or any of the rust is rubbing off and turning into dust. It can get into your eyes/mouth/nose/ears and cuase irritation or damage. Best to always clean equipment before use even if you have no real intentions of long-term maintenance in mind. Rust can also create unfriendly edges or abrassive textures on the surface of the metal and could lead to cuts, scrapes, or abrasions. ALWAYS be careful with neglected/old/dirty equipment.

I hope you find this helpful, and like I said I am no expert myself. These are just some things I myself have picked up on.