Thank you so much for your great question about painting a steel home, which made us do some research on the very interesting history of Lustron Homes.
Lustron Homes were steel ranch-style modular homes created to meet a severe housing shortage when soldiers were returning home from WWII, and were created by the forward-thinking Carl Strandlund. The entire structure was made of steel, including the framing, interior and exterior walls, roof trusses and roof tiles. The interior and exterior walls as well as the roof had a glass-like baked-on porcelain finish and your exterior color finish choices were pink, tan, yellow, aqua, blue, green, or grey and the interior color choices were beige or grey. The Lustron Homes were originally estimated to cost about $6-$10K each, not including the lot. They could be manufactured in 400 hours and built onsite using 3000 pieces and in about 300 man hours. The kitchens featured the Thor washing machine which also was a dishwasher. Items like that are what made the Lustron homes so amazing, and everything was designed to make the most efficient use of the house. Heating was a ceiling-mounted oil furnace which would heat the metal ceiling tiles and then radiate through the rest of the house, but it was a failure and most homes changed it out after completion. In the end, the Lustron Homes cost about $11,000, compared to a similarly sized and styled wood framed home at $8000. The company went bankrupt shortly before 1950 and had completed about 2,680 Lustron homes in around 18 months.
Your question about painting the interior of this historic steel home is a good one and I think totally doable if you prep correctly. First, clean the surfaces with a 50/50 solution of water and ammonia to remove any grease or soap that might be on the walls. Even in rooms other than the kitchen and bath, make sure you properly clean the walls. Dry the surfaces thoroughly and apply Zinsser's Bullseye 1-2-3 primer to make sure your paint adheres properly. Allow to dry well and paint the top coat with an oil-based, high-quality paint. You might need two coats and if you like you can use an epoxy coat finish after the paint has dried to really seal in the finish and make it super-durable. Be sure not to use harsh abrasive cleaners on the painted surfaces--try a soft-scrub type instead. On the surface, you are going to keep the original baked-on finish, and you can restore the sheen with automotive paste wax.
Happy Decorating, good luck with painting, and enjoy your sturdy and unusual historic home!