A model layout should be more than applying tracks to a base and running trains over it. There are no right or wrong answers to these questions. Your personal preferences will determine all the answers. When deciding on purpose you should consider who would share the layout experience with you. You may be part of a club or choose to run trains only with friends, family or alone.
Some people prefer their layout to emulate a particular era and or railroad.
A common recommendation is to model what you have seen or know, which also applies to how the layout will look. Not only money will be required to bring your dream into physical reality, but time. Time to build a base or benchwork, lay track, and plant scenery, then to finally run a train. Consider a small investment before taking on a larger challenge. Building a basement railroad empire can take years, even with others helping.