Hmm... why do you prefer F100 over the D80? Is it because the built and handling, or you prefer to shoot film? If your preference is the former then one option is to sell away both the F100 and D80 and get a 2nd hand D200. You'll get the same or better built/handling, and can keep your sigma 18-50/2.8dc.
Is selling your digital stuff worth it over a F100? Going FF and selling your existing stuff sounds like a costly venture. Its probably cheaper to find a 2nd hand MF 28, 50mm for your F100. I'm not too sure abt Nikon, but if there is a M42 mount adapter available, you may want to consider that, so that it gives you an option to use cheap and good old lenses for both cameras.