Unfortunately, there are only a couple of manufacturers that indicate the era when a particular model of a prototype first appeared. Red Caboose and Intermountain are two that I can think of.
As a general guideline for your selected era, for boxcars: with a couple of exceptions, no boxcars over 40'. 36'ers are also okay, wood or steel for either length, single door, doubledoor, door-and-a-half. Plugdoor cars started to make an appearance in the '50s, but weren't that common. The Athearn 40' sliding door car, Accurail wood single or doublesheathed cars are both good examples. For 50' cars, doubledoor automobile boxcars were fairly common, in both steel and singlesheathed wood (the Athearn double sliding door car with riveted sides is a good example of the former, while the Walthers car is a good example of the latter). No outside post cars, like the Railbox-style car.
For hoppers, composite (wood and steel) or all steel construction in 34' and 40' lengths, outside braced or offset side. Any of Athearn's in these lengths are good examples.
Gondolas: 40', 50', 52', or 60', in composite or steel construction. In general, riveted construction, although Concors 50'er is a good example of an early welded car. Examples of suitable riveted cars include Athearn's 50'er, Proto2000's 52'er, and Tyco's and MDC's 40'ers. Suitable composite cars would be the Tichy 52' war emergency car or the Intermountain 40' drop bottom gondola.
Tank cars: generally 40' or less, with frames and walkways around the car. Flat or slightly convex ends, but not overtly convex. The 40' Athearn cars are suitable, as are the Proto2000 cars. Single, double, or triple domes, but generally, not domeless.
Flatcars: 30'(MDC/Roundhouse), 40', 50'(Athearn), or 52' cars(Proto2000 and Walthers), riveted or welded construction.
Refrigerator cars: 36' (Tyco, LifeLike, Atlas), 40' (Athearn, Walthers, Intermountain) and 50' (Athearn, Walthers, and MDC - These are generally express reefers of doublesheathed wood construction and usually used in passenger trains.) While mechanical reefers became more common in the late '50s, they were far outnumbered by ice cooled cars.
While these are only generalizations, and there are many other suitable cars available, this should help you get started. You could also check the "Built Date" on the sides of the car, as most manufacturers are becoming more accurate in their lettering schemes.
An excellent source of information on freight cars, complete with prototype histories, is available in a Railroad Model Craftsman series entitled Essential Freight Cars, by Ted Culotta. This has been running now for at least a couple of years.
Wayne