Good work, but I have some crits.
The recent addition seems, IMHO, to be quite blocky, and looks like it has a normals problem.
Regarding selective smoothing, just select the faces you want to be solid in edit mode and press set solid - this would help on some of those cylinders. The new part seems quite blocky, and it is low poly - 9000 faces is not a lot, my current main WIP has around 20000 faces - but I am probably not an expert at reducing poly count.
Also, the legs you have seem rather skinny, have you got any references? I found one of a toy APU, it has wires crawling all over the rather thick legs.
Here is the picture I found, (cropped to remove white space, used without permission):
Sorry if you already have good references.
Apart from that, you are doing well on a detailed and ambitious project.