Well you are seeing the affects of mass production on an assembly line. There was then, and still is not nearly the variety in diesel-electric locomotives as in steamers. Steamers were often built to RR designs and specs and each RR knew what they wanted for their lines. So steamers were basically very custom compared to diesels. Also steamers were often ordered for specific jobs/assignements and the design was modified for that.
Diesels came along and once established were built on assembly lines. Options were available of course but mostly hidden. Gearing on traction motors could be changed, ballast in the frame might be changed, which end was front might be changedm but these things were basically not visible on the outside. Small differences that were visible might radio antennas/train phones, fuel tank size, Horsepower indicated by different louvres or stacks, dynamic brakes and added steam generators for passenger service would be visible but not blatantly so. And EMD perfected this with their experience on the assembly for autos. They built diesels the same way, just much longer cycle times in station. The only big differences visible from RR to RR was in fact the paint job. Pennsy freight diesels by the way should not be black, they were painted DGLE (Dark Green Locomotive Enamel) which is a very very dark green that may look black until put next to something black.
Now for Lionel, during the times you are shopping, they did make representives of EMD F units and Alco FA/FB units. These are the bullet nosed ones you mention, I think. The Alcos were aimed at O27 curves and much undersized. The prototypes were really much same size as the EMD F units. Look at them and you will see a lot of difference.
Lionel also did was they called a GP7 and GP9 which is a road switcher with the cab offset from the midde to one end. Actually they are all representive (based on the doors on the sides) of GP7s with and without brakes. Then they used the same basic design to make a "gp20" which has a low short hood and looks different from the 7 and 9.
Lionel also did a very stylelized center cab thing based on a GE switcher (44T or 60T maybe?) This is much much different than anything else in the line.
Finally Lionel did a version of EMD NW2 switcher. This is an End Cab unit that looks much different, at least to me, from the GPs or the GE unit. The end cab really sets it off.
Lionel in that era did not do Baldwin or other Alco models that I am aware of.
I will leave the electrics to someone else to describe.