Here are some thoughts/opinions on things you asked about as well as others:
- Continuation of MTH's S scale line - There are recent rumors that a group of employees will be buying out MTH, but I have nearly zero hope that even if they do that they will continue S scale products.
- Freight & passenger cars
- In S there are only three manufacturers making ready-to-run non-brass rolling stock, Lionel-American Flyer (LAF), American Models (AM) and S Scale America (SSA sold by Des Plaines Hobbies http://www.desplaineshobbies.c...22/Railroad-S-Scale/). MTH still has some leftovers and they are supposed to make the S stuff from their last catalogue before closing shop. But there is also lots of rolling stock available on ebay and elsewhere from out-of-business companies like SHS, Pacific Rail Shops (PRS), and Downs.
- For freight, SHS, MTH and SSA are all excellent. The AM stuff is great and they have a huge selection for our little S scale market, but slightly less detailed than the three aforementioned. The LAF stuff, except for the cylindrical hoppers which do not match your modeling era, is all hi-rail with trucks, wheels and couplers that bear no resemblance to reality. With some customization and some specialty products you can change the trucks and couplers.
- As for passenger cars in non-brass, your only choice for something decent is AM and some of the old SHS passenger cars which are harder to find. AM has streamlined Budd cars and heavyweights. Their web page has good pictures of these http://www.americanmodels.com/.
- Couplers is another decision you’ll need to make. Kadee 802/808 are the most common. You can also use Kadee HO #5’s which are compatible with the 802/808’s as are the San Juan ON30 Evolution couplers which are very realistic but a little big looking for some modelers. I use the Kadee 802/808’s and the San Juan Evolutions.
- There is scale only track and then there is track that accommodates both scale and hi-rail.
- If you are seeking true prototypical scale track, then the options are limited and most true scale folks actually hand lay the rail and ties themselves. If you want scale flex track, then http://www.tomalcotrack.net/ or https://www.custmtrax.com/index.html are probably your best bets. Both of them sell turnouts too. I don’t know if either of have much inventory.
- However, my strong recommendation is that you go with code 138 rail which will accommodate hi-rail and scale. There is a much greater supply of flex track in this size and they are interchangeable. Products are readily available from Fox Valley Models (FVM), MTH and the former S Helper Service (SHS). Furthermore you can get good scale turnouts from FVM and they have plenty in stock. They also make hi-rail turnouts which can be modified to accommodate both types of wheels but they are currently out of stock and being manufactured. When I eventually do my layout expansion it will be all code 138 flex track with 33" radii curves to accommodate brass steamers with scale wheels and FVM hi-rail turnouts that I will modify to accommodate both types of wheels.
- Curve radius - This really is a matter of what you want to run. I agree with Rusty though that if you do 36" you should be able to run just about anything ever made in S with the possible exception of some of the largest brass steamers from the likes of Overland and Sunset.
S scale in general
- I love S scale. The product selection is FAR, FAR less than HO or 3-rail O, and there is nearly zero new stuff coming out in S. But the size is fantastic. I initially wanted to do O, but having space to do an interesting layout drove me to look at S. I knew I didn’t want to do HO because it is too small for me to do the repairs, customizations and kit construction that I want to do.
- And regarding the selection, while much smaller than the popular scales, I have spent $thousands, and yet there is still plenty out there in the first or second-hand markets that I like but don’t own, particularly in brass. So I am not that worried about it. I suspect in 20 years there will be no new S scale coming out other than from some of the small kit makers, but I will still be having fun with working on what I have and probably buying from the second hand market, especially (and sadly) as estate sales of train collections seem to be coming more frequent.
- The reference to a Cabin Fever Auction above - Don't bother. There are less than 5 items that would be of interest to a scale modeler and all of them are modern L-AF diesels that don't match the era you are modeling.