The question is: do you REALLY want to invest even "$100 or less" in any conventional locomotive?? If your layout (now or in the future) grows to be large enough you will appreciate walkaround cab control.
A brand new Lionel starter set is about $300- and for that you get some track, a handheld controller, a locomotive with the benefits of cab control, smoke, and sound, and 3 or 5 cars. If you have tubular track, there is an adapter to add the newer set track with roadbed to your existing layout. If nothing else these allow you to enjoy your trains from multiple vantage points.
The (new) starter loco alone lists for near $200. But the sets, new ($300) or used ($150?), offer a lot of features for the money.
The locos in "starter sets" are OK- really good, for the money. Their weakness is that they have very small motors, and are NOT designed to pull more than a very few, light cars - 5 or 6, or any grades. In some cases, once the small motor dies, you can upgrade for about $30 to a larger motor. The sound features are generally designed for kids- ("All aboard for the Pennsylvania Express!!")- and a less-than realistic set of engine sounds than a $500 new one.
You can power them from the transformer you already own (in place of the set wall wart). The weakness is that the older transformers had circuit breakers that were slow-operating, and could allow a short that will smoke a modern engines' electronics in case of derailments. So add a $15 2-5 amp post-transformer fast acting circuit breaker.
Just my $.02.
