Welcome to FPN!
You spoke about the appearance of fountain pen writing.
Note that there are a whole range of nibs available for fountain pens which may affect the appearance of the writing, either choosing the nib at time of purchase or in some pens they can be changed later (Pelikan and Lamy, except for the 2000, are good for user nib changing), as well as getting your nib ground by a nibmeister.
Some pens have nibs like nails, and give very little variation in line. In some ways it's a little like writing with a ballpoint (although that's a gross over-simplification as well as heresy), except smoother, needing less pressure and a looser grip.
Many standard fountain pens with round nibs have only a slight variation in line width.
Some pens (particularly some of the vintage nibs) have lots of flexibility in the nib allowing you to vary line width with a little pressure as you write - common in copperplate and Spencerian (roundhand) writing. I've heard they tend to catch on upstrokes, so upstrokes require only the lightest pressure thus producing hair-thin lines.
Some pens have italic nibs (stub italic or stub, cursive italic, and crisp italic) that give variation in width due to the shape of the tip of the nib (roughly rectangular instead of spherical), usually giving wider lines in more vertical strokes and narrower horizontal strokes - similar to calligraphic nibs. Rounded/smoothed Stub italics and cursive italics are better for normal writing than the knife-sharp calligraphic crisp italics which I've heard in inexperienced hands can cut pretty lace patterns in paper instead of writing.
The Pilot 78G (sounds like a good choice) has normal round nibs (not a lot of line variation) for fine and medium nib sizes, but the broad nib version is actually what many other manufacturers would call a stub (or italic or stub italic) nib, like writing with a little flat piece of metal rather than a round tip.
This might possibly be more what you are after if you want that line variation, although you might prefer e.g. a fine nib for smaller writing.
The Lamy Safari, a great inexpensive beginner's pen as well as being popular as a workhorse pen rather then "pen jewelry", also has the option of readily available italic nibs which are quite usable (I recommend the smallest, 1.1mm, for normal writing). The advantage of the Lamy Safari is that it is relatively easy to buy and exchange nibs yourself if you want to try something different, while I suspect the Pilot 78G will be more of a case of buying the pen with the nib you will then continue using.
Useful reading:
http://www.richardspens.com/?page=ref_info/nib_beyond.htmOf course, with fountain pens, you aren't limited to boring blues and blacks

- you can write in a whole range of greens, browns, etc.
Going back to the appearance of fountain pen writing, many of the non-black ink colours often show a unique characteristic of fountain pen writing - "shading" - due to more ink pooling near where you lift the nib, you get lights and darks throughout your letters. You might also get the same with some pens and some blacks - shading from deep black to more of a dark grey.
From my limited experience I've seen more shading with lighter less-saturated inks, and more shading with non-cursive (non-joined) writing styles with more pen lifts to get those shading effects at the ends of lines.
Lastly, of course, a fountain pen itself can't automatically improve your writing, although it can help you think about it more as you write so it can have a beneficial effect.
Regards, Myles.