Yes, unfinished (beams and ducts exposed) ceiling even with insulated, drywalled walls, and carpet on the floor is still an "unfinished" basement in some locations. But it's not a hard and fast rule. Just depends on the county or municipality that sets the taxes. And the rules can change over time. Painting the "unfinished" ceiling black kind of makes it disappear -- it's an architectural trick sometimes used in fast casual restaurants and strip mall stores. Other colors are possible, too. Matte colors are usually preferred because you won't get odd reflections off of joists and ducts. Even if it doesn't make a difference on taxes, sometimes it's a desirable look because light fixtures can be dropped anywhere, and with few or no ceiling reflections, light tends to be projected exclusively downward or outward depending on the fixtures for more of a "stage lighting" effect. Sometimes that can be useful on a model railroad.
Drywall ceilings look the nicest, but they reflect more sound back into the room -- so track noise may be louder. They're also a lot worse for a mess if you have to open it back up for repairs. Drop ceilings give access to whatever is above, and they provide some sound absorption.
When I was house-hunting, (Or was it basement-hunting?) I looked for floorplans that spread out a bit. Ranches can be great, along with some "1 1/2 -story" houses with a partial upper floor that's inside the roof gable - most of the house is still spread out over a decently sized basement. Houses with the garage partly or fully underneath mean that the garage is intruding into the basement -- you may find there's not much real "basement" at all beyond what's taken up by the furnace and water heater area. Pre-finished basements that are broken up into exercise rooms and rec-room areas aren't always bad. With an around-the-walls layout, the wall separating the rooms can become a divider, with different scenery on both sides. An interior wall is usually easy to cut a hole through (if it's drywall or paneling) which can then be embellished with a tunnel portal for the train to go through to the other side.
