Kim
I am partial to things that do not move when machining them. So I would use either hot rolled or 4140 or 1144(stress relieved stuff) where things need to stay put ie fluting a side rod or making linkages. Hot rolled has the disadvantage of not being dimensionally accurate (rounded edges)so you need to buy larger size and mill down ie if you need a 1 x 1/2 a 1 1/8 x 1/2 would need to be purchased. Use as little 12Lxx as possible as it rusts. The use of steel in places where Kozo calls for brass might be better from a strength point of view but this is a small model so I doubt it would actually make a difference. If silver brazing is necessary then stick with the brass. Stainless is good for looks sometime, like the side rods nice and polished, but if you do not intend to polish them bright then just steel. Making fasteners from stainless has advantage of nice looking and removal later with no rust.
Remember when making the copper boiler NO BRASS for anything ie bushings, manifold etc, use only bronze. If you opt for a steel boiler, same thing no brass.
In a model this size you are not using that much material so cost is usually not an issue, use the material that will make your life easy.
Hope this helps
Bob