There's a Williams (Samhongsa) brass N&W J 4-8-4 on Ebay right now for $300. It's conventional, but it's a beautiful scale stream-lined engine that doesn't have a lot of stuff sticking out that may break off.
It can't get much simpler than this. Engine, rolling stock, track, and a transformer is about all you'll need to put a train under the tree.
When I got mine I converted it to PS2 command control, it ran like a fine watch and sounds were great, but you would have to go up to a DCS system to have all that.
I have since converted it to battery powered, remote control.
On the down side...it's a 13lb engine/tender, don't let the little ones try to lift it. But it sure is purty:
