The build of an instrument is dictated by the scales and tunings common in its culture or genre. These provide the 'target' towards which builders innovate.
In the case of the harmonica, innovation has been recent and conscious: an attempt to ease playability in various genres, facilitated by manufacturing advances.
In this sense, rather than trying to understand the instrument, listen to music from the genre it was built for, and what emotion or 'groove' players are trying to express.
The blues harp, for example, is a simple instrument built around the blues scale. If you understand this scale and are familiar with the blues 'groove', the layout of the instrument is more or less self-explanatory. Trust your ear, and all else will follow.
Orchestral or jazz harmonicas, on the other hand, are built around the notion of free play in any scale ('chromatic' harmonicas), leading to additional complexity - and challenges. High amongst these are the issues of coordination and breathing control, and, if with other classical players, reading music.
One of the more recent harmonica innovations are the 'Irish' harmonicas of Brendan Power. In themselves a simple advance, but vastly easing fast, rhythmic play in 'modal' Irish tunings.
That said, there are many types of harmonica. If nagged by the buying itch, rather than first looking for a harmonica, perhaps choose a musical genre that appeals, then look for the matching instrument..
Who knows. Maybe you'll discover an unexplored niche (Balkan? Chinese? Arabic?) in the world of harmonica design. ;-)
Trust. Your. Ear.
Postscript: for folk or traditional players, reading music is in many senses unnecessary ballast (and very few folk virtuosos can read a score). Try instead to play along with a slowed tune (using VLC or one of the many other audio/video tools), using an instrument of your own in the correct tuning (if necessary, ask another player for help getting this set up). You may be pleasantly surprised how quickly things fall into place.
A couple of weeks learning by ear and you will be playing your first tunes independently, and will have begun to develop the 'tunings intuition' that helps you hook up with others in a jam.
Try, on the other hand, to learn to read music and you'll be busy for the next year or two, but will have picked up little of the tension, speed and dynamics of the original recording. You may also have missed an opening in a local band..