I think that an excellent general theme of advice among the posts is, that if you buy a loco for $100 or less, there is going to be a high chance that it will be an under performer, or have a hidden problem, or will work out so that you are just not happy with it. So, there goes your $100.
On the other hand, if you spend just $50 more or so, and get into the used $150+ range, you have much more selections to choose from, and can generally get a much better locomotive.
Also, I understand that you are looking for a steamer, but my experience over the past 18 months is that mid-size to large size diesels are virtually trouble free compared to steamers. Two trucks, fore and aft, plus dual can motors, plus really nice electronic horn and bell sounds, are what you get. Steamers, though beautiful, may have the issue of small bouncing fore and after trucks, often aching for derailment, smoke units that work for a while or not, whistles that may or may not really slow down the engine, drive rods that may be or become bent, causing wheel binding, and a host of other mechanical issues, and often the extra train weight of a tender.
If you are the type of person who enjoys analyzing and solving these types of issues, then steamers are for you.
If not, then consider the diesels, or limit yourself to steamers like the post war Lionel Scout Engines, that are very inexpensive, virtually trouble free, and bullet proof, or used inexpensive Lionel starter set engines.
Hope this helps.
Mannyrock