Yes keep looking. It makes no sense of course to have to rip out walls and rooms in a finished basement as noted already, but I think it's worth pointing out that an unfinished basement it not the only option.
My basement was finished except the laundry room and it had a big enough finished room in it that I could use with minimal preparation. With in a couple months of moving in I had the room painted with sky blue paint (over the awful dark biege color) and was building benchwork. So if you can find a basement with a drop ceiling and a large room with sheetrock already in place, thats a bonus really. Otherwise yes, large open basement can be "cart blanc" for a model railroad.
Even if you expect to be in that house for a long time - 10+ years, I'd still build the benchwork in a semi-modular or sectional format so you have the ability to move, remove it in part or whole later. It saves using a sledge hammer or sawzaw or dynomite later. Options are always good!
Rio Grande. The Action Road - Focus 1977-1983