First question: yes, it'll replace both of those bikes. It may not be quite as nimble as the DR650 and it may not cruise the Interstate quite as steadily as the DL1000, but life is full of tradeoffs.
Second question: the tanks holds 4.2 U.S. gals, though people who have run it dry have said that they've squeezed in a scosh more. Requires premium gas, but can be remapped for regular (by the dealer) at the expense of a couple of mpgs and hp.
The confusion about octane ratings may have something to do with the difference in the way it's calculated here in the U.S. and Europe.
Here's a recent thread about tank range: