Maxi,
Any of those cars you posted would be fine on a 40's to 60's layout although you have to narrow things down a bit. For example, the first two cars would have still been in use in the 60's but the plug door Maine Central car wouldn't have come into use until the very late 60's.
It looks like the first two are Accurail kits and the Maine Central is an Athearn or Roundhouse kit. You can see the main differences between the two types of kits. The first two have separate ladders and grab irons and the details, like the roofwalks and stirrup steps, are finer. The last car has the ladders and grab irons molded on, the stirrup steps are thicker, and the roofwalk is also thicker than scale. Now, how much of this would you notice five feet away? Practically none unless you got an old Tyco or AHM Car where everything was two or three times out of proportion for scale.
Since you are wanting to build up a freight car fleet, I suspect your money will be rapidly depleted by buying Accurail. Atlas Trainman cars are a good deal for the money but I will go with Paul and Josh's suggestions. Try for some of the older Athearn "Blue Box" (so called because they come in a blue box
) kits as your first choice. It's often possible to find sellers on e-bay who are letting them go in the $5-$8 range and they look and run good. I'm not a great fan of the old Roundhouse line of freight cars but there are some that are unique that only Roundhouse built. With some care, they will also be nice rolling stock, and they can also be found at very reasonable prices on e-bay. Save the big money for special kits that you want to use as foreground detail or where Accurail is the only make of the model you want. I would guess that 80% of my freight car fleet is Athearn Blue Boxes. I'm slowly replacing these with Accurail, Kadee, Branchline, and Atlas freight cars but I expect this process will take several year because of budget and the amount of time it takes to build each kit.
A real nice compromise if you can find them are 4 pack Yardmaster kits from Branchline. The details are molded on but they are very fine and they assemble almost as fast as an Athearn BB kit. I just got a 4 pack of NKP 40 foot boxcars from e-bay for $27 so keep an eye out for those.