I'll second that on the Keystone Locomotive Works castings. If you take your time cleaning them up, they look great. I built a batch of them to detail a planing mill. Now if I could find time for the building ...
One other tip: these things will be inside, so some color is needed on them. Machinery from this timeframe was mostly done in machinery gray (like SP light gray or light haze gray) or in machinery green. I used those colors plus some Model Masters colors like wood for the live rolls and jet exhaust (yes, that's paint color) for small details.
Good luck with your mill!!!
--Stu--