Welcome to the S Scale forum.
Well, your questions would have been a lot easier to answer about 3-4 years ago. S has undergone some turmoil over the past few years, due mostly to what can politely be described as "outside forces" beyond the control of the manufacturers.
You might want to look over the thread that's in the top section of the forum:
https://ogrforum.com/t...questions-on-s-scale
But, I'll try to give a semi-detailed answer by manufacturer.
American Models:
Locomotives are available AC w/Flyer compatible wheels and couplers, DC w/Flyer compatible wheels and couplers and DC with scale wheels. AM has a scale "snap lock" dummy coupler with the scale DC units. Some have been also having Kadee couplers included. It's up to the modeler to install them.
If you want sound, the AC is the way to go, but the AM sound systems are very basic diesel sounds with horn and bell. Steam sounds are whistle and bell, whith chuffing supplied by the smoke unit piston ala American Flyer.
AM has track similar to Atlas HO snap-track. There is no molded ballast. It will hold together on a hard surface like plywood or hardwood floor, but could be a problem on a carpet.
Lionel/American Flyer:
Things got interesting with American Flyer over the past year. With the introduction of the scale SD70ACe's, these locomotives will run using conventional AC, Lionel's TMCC/Legacy operating system, DCC and conventional DC. The SD70's also have full sound, access to the sounds depends on which control system is used. It's all internal to the locomotive and dependent on the code on the electronic boards. Earlier scale locomotives like the first run U33C's are AC or TMCC/Legacy only.
As I recall, all following Legacy equipped American Flyer will come fully coded for operation with any of the above mentioned systems.
Lionel hasn't quite settled on the matter of scale wheels, yet. It seems to change from catalog to catalog. The new GE units come with traditional Flyer wheels, while scale wheels are supposed to be offered separately.
To further add to the confusion, Lionel's cataloged three steam sets with a 2-8-4 that will be conventional AC: Polar Express, C&O Passenger Set and Nickle Plate Road freight set. No price has been given yet.
Lionel's S FasTrack is currently only available with straights and 20" radius curves. Turnouts are coming.
MTH:
The great unknown. Unit something actually shows up, the only information we have is the MTH catalog.
MTH recently purchased the S-Helper Service tooling and this is what the S Scale line will be based on for now. MTH is in the process of reworking the SHS tools to suit their manufacturing technique. As you've noted, nothing from MTH is available yet.
The F3 diesels are supposed to come equipped with DCS, which will run on AC, DCC and DCS. I'm not sure about DC. Scale wheels are also supposed to be included with the F3's.
Under S-Helper Service, freight cars came with Flyer compatible wheels and couplers installed and a set of scale wheels were included with each car. It was left to the purchaser to buy his own Kadee or other couplers.
MTH freight cars will come with Flyer compatible wheels and couplers. Scale wheelsets will be available separately.
If there are no changes to the S-Trax system it should be a reliable system, but once again, we won't know until it shows up.
Finally:
I wish I could point to something and tell you "Yeah, that should get you going," but to be honest, the availability isn't there right now.
If you're still with us, I would suggest sticking with the larger flanges and couplers compatible with American Flyer and AC power for starters. That should give you the widest choice and most flexibility of what's available right now. And unless you know of someone with a cache of old S-Helper Service S-Trax, look to Lionel's S FasTrack for availability.
Rusty