Well Manny, I can't tell you what you should purchase, but you've posted before about wanting locomotives that look good with smaller sorts of rolling stock. That K-Line S-2 is a perfect candidate. I should mention the K-Line S-2 has origins with MARX, and you can still find the MARX S-2's for sale. A much simpler constructed loco, but that was one of the hallmarks of MARX products. As mentioned above there's also the improved RMT version, but you likely won't a deal on that one.
Now, as of the last year for whatever reason (probably the lack of train shows), train prices in general seem to be much higher (as you may have noticed) than a few years ago. Increased shipping charges certainly need to be factored in to any train purchase.
But years ago, I could get K-Line S-2's for $50.00 or less. I've gotten them at train shows for $35.00. So any changes or repairs I might have to make weren't so unjustified. There is a possibility that you could place the engine on your track, and you'd find it runs just fine. BUT if it didn't, correcting the derail problem is not all that hard, as I described above.
When I fix the open pilots on my S-2's, I use self adhesive weights on the underside of the frame, between the sheet metal steps to adhere my new pilot cover, so that adds a little more weight the front of the engine. (Yes this is a repaint with other alterations and details added).
And by the way, your idea of using the frame and motor from a NW2 won't be an easy job. The truck from the NW2 mounts to the frame from a single mounting pin from the top of the motor truck with a "C" clip holding it to the frame. Without a second truck, one alone will not be stable. You'd probably need to make an entirely new frame that the motor truck can be securely mounted to so that there is no wobble of any kind.
Now if you're referring to an MPC or postwar version of the NW2, that might be a bit easier because the motor sticks up through the sheet metal frame. Still might be easier to make a new frame mount out of thick styrene or basswood/plywood. Personally, I'm always open to a train challenge, so good luck on your project should you attempt it.
The guy you saw the video from is an acquaintance of mine who also does quite a bit of kitbashing and repainting as I also do. He used the motor assembly from a MARX E7 for his BEEP project.
