Hi,
My approach is to buy good running and complete kits of anything close to the unit you want, preferrably unassembled, and rework the kit to suit. You can but alot of Athearn Blue Box kits from $20 to $25 that are complete, buy a spare shell of the right road or strip it and repaint, and be far ahead if you want. The price of the frame, drive mechanism, and detail parts is far higher than a NOS Athearn blue box on eBay.
I have about 30 SD40s and GPs that are in progress of being rebuilt to my standards starting with $20 Athearn kits. Detail kits from places like Details West take it to the next level.
Bottom line: Don't build from parts, kitbash is faster/cheaper!
GB
P.S. I just had a NOS Athearn GP38-2 delivered from eBay for $25. Once details, new handrails, tune-up, and a re-wire are done. It will be a great unit.