Three Hand Planes to Begin With
Bench planes are sold as a numbered range that looks like a set of near-duplicates differing only in length. They are better understood as three jobs, and once the jobs are clear the choice stops being a matter of collecting sizes.
The first job is removing material quickly from a rough or twisted board. This wants a relatively short plane with a blade ground to a pronounced curve, so it takes a deep narrow shaving and does not care about the surface it leaves. Short is the point: a short sole follows the existing hollows instead of bridging them, which is exactly what is wanted when the aim is to get material off rather than to make anything flat.
The second job is making a surface flat and an edge straight, and it wants the opposite. A long sole bridges the hollows and rides the high spots, cutting them down until the surface stops being a series of high spots. The length of the plane is doing the work; the blade is nearly straight with the corners slightly relieved, and the shaving is thin and full width. This is the plane that makes a joint close along its whole length rather than at the ends.
The third job is the finished surface, and it wants a short plane again, but for a different reason. A smoothing plane takes a very fine shaving from a surface that is already flat, and the short sole lets it work locally, following the surface rather than reshaping it. The mouth is set tight, the blade is sharpened straight with the corners eased so it leaves no ridges, and the result is a surface no abrasive will match for clarity.
The practical consequence is that the middle plane is the one to buy well if only one can be bought well, because flatness is the property that cannot be faked by any other means. Roughing can be done with almost anything. Smoothing can be substituted with a scraper or abrasive at some cost in quality. Flat has to come from a long sole and a straight blade.
Everything else in the range is a refinement of those three positions, and there is no point acquiring the refinements before the three jobs are being done reliably with three tools.