What MEC407 said.
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Assuming you are not into model railroading, a bit of maybe relevant information. In HO scale, a modern locomotive like an ES44 or AC44 or C44-9 would be about ten inches long. In N scale, about 5 and a half. These are by far the most popular sizes for model railroading, and so (economies of scale and all that) the cheapest: a plastic model in one of these sizes can probably be obtained for significantly under $50. At that price the modelling won't be perfect: some of the grab-irons may just be moulded on, the handrails (particularly in N scale) will be too fat, etc. At the other end of the price range, custom painted brass models with exquisite detailing will be several hundred dollars.
Other scales exist, but are less popular so probably more expensive. There is probably an O-scale C44-9W (about 18 inches long) in the several hundred dollar range.
I'd recommend a visit to a hobby shop / model train shop as a next stage in your researches: manager will know what's available and at what cost. ... For display, you obviously don't need the motor inside: many model locomotives are marketed in unpowered ("dummy") versions: slightly cheaper at the low end of the price spectrum. (At the high end, I suspect that the motor is a small enough component of the total cost that the manufacturers may not bother offering a dummy option-- this is the sort of things hobby shop manager would know.) ... I think there are options (either out of the box or with a very small amount of screwdriver work) as regards couplers: a display model can have more "realistic" (= closer to scale appearance) couplers than one designed for operation on a model layout.
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I hope you get a model. I've long been tempted, and one of my daughters once offered to paint a model locomotive for me. (I think a General Electric U18B locomotive painted -- alternative historically -- in New Haven Railroad colours would be very nice.)
