Another possible approach to a task like this is to make heavy use of material-and-texture nodes.
Basically, “what’s most important, in selling your ‘steel’ texture as authentic, is the scratchy pattern of lines … not (particularly) the color.” So, maybe you could manipulate that image to a very high-contrast, black-and-white “pattern of lines,” and use this as a mapping input, say, to a bump-map. Maybe you also use the map, this time as an alpha-input, to mix two colors … the base color of the steel and a second color that will be seen as reflected highlights. In this way you can apply “that scratchy pattern of lines,” in various ways and to various degrees of intensity, to almost anything, along with any other factors (specularity, etc.) that you think might heighten your intended visual effect.
Some of the effects will qualify as “materials” … “the base color(s) of the object” … while others will qualify as textures … “the effect upon reflected light.” You will likely use a combination of both, and use nodes in both cases.
The entire "nodes’ system encourages you to experiment, and to build up visual effects out of simpler components. It also encourages you to “see, critically,” your progress so-far and to then decide what new spice to add to the brew as it is cooking.
If you simply look upon materials and/or textures as “digital shrink-wrap,” you’ll probably never be satisfied with what you get. This is a computer, and you need to feed it data and (by means of nodes) describe what it is to do with it. Let the computer truly compute what’s happening on the surfaces of these objects, and not simply “blindly take it from a PNG.” (Cuz that’s when you really “start cookin’ with gas!”)
I think that BlenderGuru’s Basics of Realistic Texturing tutorial is an exceptionally fine one.
Also note that Cycles, “by the intrinsic nature of the beast,” is quite different from BI in its handling of both materials and texture. Nevertheless: (a) you are not limited to using “only one renderer” in the construction of a scene, and (b) the node-based principles are still applicable, especially in the case of Cycles, which uses nodes for everyhing.