Welcome!
I find that the people who enjoy collecting little cars the most are usually people who like Real Cars. My son and I both collect diecast cars. He's gotten to the point where he has his own taste and will pick some of the wilder Hot Wheels from time to time. However, since I take him to car shows and hot rod nights, he sees and recognizes different types of muscle cars. If he doesn't recognize a car, he asks me about it, and we have fun researching it on the internet together. We also belong to a diecast club and go to the meetings to race and trade together.
Ditto to everything already said; Don't think of the collection as an investment, keep track of what you have, "play" together.
As for actually accumulating the cars, the first and best way is to find what you can locally so that you don't have to pay for shipping and handling. However, if you are looking for some older JLs, I have several places where I go online. These guys usually have good (or at least fair) prices on older cars, and I've been able to fill in several holes in my collection with them instead of resorting to eBay.
tinywheels.com
67diecast.com
1stopdiecast.com
Hope this helps. I'd be interested in your progress. Sometimes it's hard to complete for the attention of a young boy, what with Yu-Gi-Oh, TMNT, Transformers, GIJoe, etc. I feel lucky that my son will spare some time for me via the cars.
-Brad