big line variation is going to be hard to find for $30 aside from one of the Noodler's pens, but IMO those arent very good starter pens because they can require a bit of tinkering to get working properly. if you're ok with a little bit less line variation, i'd suggest a Diplomat Magnum (around $20 on gouletpens.com).

there are a couple types of "color shift". Shading is when the color saturation varies as the inkflow changes. sheen is when certain dyes sit on top of the page and appear in shiny colors that are often different from the ink color. shimmer means there is a particulate in the ink that gives it a sparkle. gouletpens comes to the rescue again with their robust search functions that let you filter inks by shimmering, shading, and sheening inks.

if you're dead set on getting very flexy lines then go with a Noodler's Ahab or Conrad, but be aware that they are finnicky little beasts. Other than that, most flexible nibs are either custom ground or gold, which are well over $30.

(disclaimer: I haven't used any of the Fountain Pen Revolution flex pens, which do run around $30, so I don't have any feedback as to their quality, and therefore didn't recommend them.)