The Jegans are not released as part of the F91 line because they were already issued as part of the Char's Counterattack line, which only had the plain Jegan, no A or B type add-ons.
The Guntank R-44 is the smallest kit in the F91 series, but it is also the only one that transforms. No accessories.
The F91 is the only kit in the line to feature System Injection, with parts pre-molded in multiple colors (for example, the "F" and "91" on its shoulders are molded in red even though the shoulder parts are white) so no painting is necessary. It is also the largest kit in the series box-wise, because of all the accessories it comes with.
The Heavygun is basically the F91 era's GM, with the G-Cannon being the Guncannon for the era. G-Cannon comes molded in apple green colors, I'd recommend priming it before you repaint the whole model. The Heavygun has more accessories than the G-Cannon but less than the F91.
You should check out some of the Crossbone Vanguard kits... they are nice designs.