Athearn makes the SD75I, Kato makes the SD80MAC. Neither are compatible with one another because of the difference in length.
I don't know what to tell you about the cab, but the cabs are available as separate parts from Athearn. I'm sure with some sheet styrene, a razor saw and some Bondo you could put together whatever you imagine.
Winterization hatches are available from a few different sources. Try
http://www.detailswest.com/catalog4.htm for starters. I also like the extra ones that come in some Life Like locomotive kits (usually in the undecorated models).
I use Scalecoat II paints exclusively, so if I wanted an aqua color, I'd probably find a blue and a green (
http://www.weavermodels.com/page11.html) to start with and experiment until I got a color that looks right. Use eyedroppers to mix paint one drop at a time keeping track of the total of each color, of course, until a proportion is found that matches the color you need. Then, mix a larger amount in the same proportions so you can paint your model.
