A couple points: The 646 and 2046 are pretty much the same loco with different numbers. The 736 has the same super-structure (boiler shell), but obviously with an 8-wheel chassis.
The 1615 is nice, but IMO the 1656 is the best four-wheel switcher Lionel made, until recently. All of these locos are a little top-heavy, so keep your speed down going around curves. At train meets you often see them with broken marker lights.
Another interesting possibility is the prewar 201 or 203. This was the same small switcher body with a six-wheel mechanism. They're kind of rare, so you would have to be lucky to find one for ~$200.
