Collecting JEP trains is an excellent idea, and i will try to answer some of your questions.
Freight and passenger cars with with hook and wire loop were made between 1933-51 and have 8 wheels
Cars with the small hook at each side called automatic couplers were made from 1952 to 1966. Freight cars now have four wheels and passenger cars still height wheels.
About freight cars, they are nearly all identical during all the period production from the thirties to the end except the couplers, the wheels. The litho is the same, except of course a great number of color variations but they are more for the specialised collector.
More difficult for passenger cars, many color variations, the most comon are the blue and cream pullmans which has stay all along the period. Some are more comon, blue and cream pullmans in every lenghts and with different couplers are easy to find. The classic passenger cars all steel are also comon and may be founded with the two types of couplers. Also some color variations with black roofs are harder to find.
One thing to consider is to buy a book on JEP. There is one for sale in your country here on the auction site
http://www.ebay.com/itm/JEP-19...a:g:thwAAOxyRhBSsxUp
It would be much more easy to discuss on a specific model. I have done the same for my Lionel and AF collection buying all Greensbergs, Doyle and Schuiller books. The book is in french but if you have problems to translate i can help a little, all models are described and not hard to understand.
Price is not easy to discuss but a little base is 30-40 for near all freight cars, same thing for common pullmann passenger cars and after much more for the longer models.
I have taken some pictures of the book so you will have an idea. Sorry for the quality i used my phone.
I hope all of this, even if it is very simple will be of some help.
Daniel