It's good you get on Cycle directly. I hope UV-unwrapping is not too much of a problem (most of the items in your scene are rather flat or boxes, so it shouldn't be too difficult). For UV-unwrapping, it's generally best practice to mark the seams yourself and then do the unwrap (as opposed to using one of the automates unwrapping method), then it's more convenient when you have to make textures ( the writings on the wall, probably the yellow & black stripes are textures you'll most likely have to do yourself). We can see about that later.
Now, for adding grime, I usually recommend to make grime textures with a combination of a/ A procedural or a grime image texture b/baked Ambient Occlusion.
- To get a procedural texture for grime (using nodes in Cycles materials): Add a Noise texture, connected to a Color HSV node, connected to a Color Brightness/Contrast node. In the HSV node, set the Saturation value to 0. You now have a base for building a grime map: In the noise texture node, you can set the scale of the fractal pattern, as well as the Detail option (I recommend Details values above 10). Then with the Brightness/Contrast node you can enhance the difference between clean and dirty areas, or make the overall material somewhat clean or somewhat dirty. Just play around with the settings. For more control over your material, you can also split the exit of the HSV node to 2 different brightness/contrast nodes: one would affect diffuse color and one would affect the Fac. value in the mix shader between your diffuse and your glossy shaders.
- For image textures, just scrounge around cgtextures.com. Good sources of grime textures can be found in the Concrete, Grunge, Metal, and Plaster sections. I advise you retouch these images before importing them in blender as grime maps (namely: make the images square, maybe retouch the colors or set a greyscale). If you choose an image texture instead of the procedural, the way you get a brightness/contrast node after that works pretty much in the same way as above.
- Now, for Ambient Occlusion: the idea is to get a greyscale map of "free space" surrounding your object. When baking Ambient Occlusion, if there's nothing in the vicinity of an object's surface, it will be bright, and if there are objects in the vicinity, it will get darker in relation to the size and distance to the surface. This can be used as a way to make dirtmaps, because it correlates how objects tend to get dirty (open surfaces are clean, narrow crevices get dirty). Now, to do it:
I recommend you split your working environment in several subwindows, with at least your 3d view window and a UV-image editor window (usually I have a third window opened for material nodes). Select the object you want to work on on edit mode, and see how the UV unwrap is made in the UV image editor. Now, save, and go to Blender Render as a render mode (leave Cycles for a few minutes). In the UV image editor where your UV unwrap is shown, create a new image (the resolution depends on how detailed you want your results to be - 1024 x 1024 is usually fine, but don't hesitate to make it bigger for very large textures). The image should be black. In the World tab of the properties panel, click on Ambient Occlusion and in the "Gather" subsection, you can tune the attenuation distance (which is in relation with the objects surrounding the one you're working on). Then, make sure you have all the object which could have an influence on the grime texture be present in a visible layer. Once that's done, go to the Render tab of the properties panel, all the way down, check out the Bake section, select "Ambient Occlusion", then click the "Bake" button. You should have your Ambient Occlusion map being gradually baked in your UV image editor section. If the results seem very odd (an open surface that has a black ambient occlusion, for instance), watch out for the direction of your face normals (you may have to recalculate them or Flip direction). When your Ambient Occlusion map is baked, Use Save As image in your UV image editor. Voila! You can return to Cycles render afterwards, and use your ambient occlusion map as an "Image texture" in your material nodes.
Once you have a grime texture and ambient occlusion, a good trick is to mix them with Color Mix node, but you change "Mix" for Multiply, and set the Fac value to 1 (or near 1). Then you use the resulting mixture as something to mix in your diffuse color, and in the Fac value of the mix shader between your Diffuse and Glossy.
I hope that helps, and I hope my instructions were clear enough to follow
Then, I think it is rather wise not to exaggerate the bumps to much for the rust. Something else to keep in mind: you'll have to be very careful about the scale of the objects (or even bumps) in a scene, because the original images can be misleading. Because the original game was set in such a low resolution, many objects were actually scaled up considerably to make them recognizable on screen. But now that we are in HD, it doesn't make sense to keep some of these objects at original scale. 2 recent examples: in the "man in pipe" scene, if I had kept the soda cans and beer cans as large as in the original scene, the cans would be about 40 cm tall and 15 cm in diameter. In my item shop scene, the original scale of a tea cup was actually 38 cm tall and the same in diameter. These features need a rescaling job. You may not encounter many things like that in your scene, but watch out for these. For instance, The dimensions of the ladder (width, size of ladder bars, etc) may need a revision. The question is: "how do I figure what should be the proper scale of objects?". There are 2 basic methods:
- Generally, large objects (bed, door frame) were made with a logical scale in proportion to the characters. Door frames and beds give you a pretty good reference point for defining the scale of a scene (generic doorframe around 2.10 m, generic mattress around 2 m long). If a tea pot is as high as a quarter of a doorframe, you know something is wrong.
- See how the size of character models compare with the size of your objects. To that end, I strongly recommend you mod your game using Kaldarasha's character models (See Graphical Mods in this forum), which have more realistic proportions. You can estimate the scale of your environment given that Cloud is 1.73m tall.
That's all I have in mind for now, and I hope it's not too much to digest