Similar to Seaboard Streak's, I would have one of those steamers or possibly a
4-6-0, 2-6-0, or a 2-6-2, three freight cars (to keep the set practical and "affordable").
a hopper of coal, a box car, and a reefer, and follow that with a combine coach caboose. This would allow freight runs and caboose only passenger runs, in other
words, get as much play value out of the set as possible. I might even add in another
coach for that reason, giving the buyer a freight and a passenger train, like Marx used
to do, in the same set. SS's track choice seems good, but I would include a transformer, and, again, nothing else, to control the cost. Also to control costs,
I would offer it as conventional (with a future option for an plug-in modular upgrade at more cost).