How much fuel range do I need?

The bikes should be able to go seventy miles without refueling. And be sure to realize that half of that distance will be at race pace, not commuting.

In practical terms, the only bike that we've seen that's needed auxiliary fuel was the BMW G450X. That bike has a tiny 2.1 gallon tank. There have been other people run out of gas, but due to either being too smart ("I'll save some weight by not carrying a full load of fuel.") or too forgetful ("Crap, I forgot to top off at the last service!"), not due to limits of the bike.

Any regular mid-sized dual sport will be fine. The larger bikes (GS1150) can easily carry way more fuel than needed. The smaller bikes, 250cc, take a look at the rally schedules and double check the distances.