The quality of Post War diesels peak around 1952 with few exceptions like the FMs, steam didn’t change too much over the years other than tenders came with increasingly less detail. Later diesels came with stamped vs cast frames and more cast on vs applied detail. I find the best deals at local train shows vs online dealers. Also check your local train stores. They may charge a bit more but you will know it works when you get it.
One nice thing about Williams is they copied the best version Lionel produced. NW2s and Alcos have diecast frames. F3s have all the grabs and screens of the early F3s. Steam engines have 12 wheel vs 8 wheel tenders when Lionel used both plus all the grabs of the early versions.
I am glad to hear you have not ruled out American Flyer, below I have added a couple websites to check out. The first one is a great site showing about everything AF ever made, and you can pick out models you like. The second site is the NASG, lots of information about S-Gauge and some photo’s and videos of our train show displays, and member layouts, and upcoming shows that may be in your area.
As for purchasing, I have had great luck wih eBay, study the photo’s read the listings very carefully, and ask questions of the sellers. Search for completed sales of the same item you are looking at for additional guidance on prices.
Greenburg also puts out AF price guides, and I have a book, called standard catalog of American Flyer Trains, by David Doyle. tHis book shows photos, discusses variations, and provides prices as well.