There is an S Scale forum here which is all facets of S gauge. Plenty of info and many to help and answer your questions. As far as a reliable AF knuckle coupler workhorse engine price from low to high would be 303 Atlantic, 293 Pacific, 313/316 PRR Pacific, 326 Hudson and finally 336 Northern. All are relatively easy to service, smoke, choo-choo, and are lighted. All are diecast except for the 303. Welcome to the club.
That is the info I was looking for