Mason,
It sounds like you want your layout to center on the Delaware Lackawanna. That is a unique railroad which I have not seen modeled and will offer the opportunity to make a great layout with a few possibilities. I do not recall any O scale manufacturers offering that paint scheme, so you will have to repaint locomotives yourself or find someone to do it for you.
I did some quick research on the DL locomotive roster and saw that the DL had one C-626 and two M-626's. I do not recall any of the O scale train manufacturers making those models. Lionel produced a C-420, but not in DL livery. Since this is one of the locomotives you inquired about, in your situation, I suggest moving to the top of your to-do list finding and repainting a C-420. While you are in the process of doing that, you will have time to reflect on and plan your layout. Once your dream locomotive project is complete, then move on to building a layout, buying track and buying rolling stock. Here's the reasoning: You will enjoy being inspired by your locomotive and using it to test your layout more than building a layout first and wishing you had the dream locomotive to run.
DL had six C-420's, but the majority of the locomotive roster was filled out with the RS-3, RS-11, C-424 and C-425. I bring this up because if you do plan a DL themed layout, you will want to eventually add them for variety. All of those locomotives have been produced, but you will have to find them on the second hand market and repaint them. (You will need a minimum of O-36 if you go after the AtlasO C-424/425.) For switching, the DL used an SW-1, SC, and S-6.
Before we can begin assisting you with your layout, we are going to need some information. Let's start by defining where the layout will be built and the dimensions of the area. The track plan and layout will be defined by the area, so try to be as specific and accurate with this information as possible.
