I"d recommend the F2 over the K......more capacity and has the flexibility of different size corn heads, plus bean heads up to 15 feet wide. Either one is easy to adjust to get a clean sample, and maintenance is simpler compared to other brands.
Older corn head are orange, come in various row widths. Newer heads, since about 1975, are called "black" heads, and are designated "adjustable" heads. (A438, A430, etc.) Row widths can be set two inches wider or narrower than the base width.
To get an F2 with the long separating shoe you need to go by sn 39201 or above. That would likely have the hydraulic unloading auger as well. F2 diesel would have the turboed 96 hp 4 cyl engine.....burns about 3 gph.