Eggss, it sounds like you don't have much off road experience. Depending on your natural abilities, you may find going off pavement on your DL difficult. I suggest taking a class designed to teach riders off road on big bikes. Or, your idea of getting a smaller bike is a good idea if you want to learn on your own or with friends. You could pickup a used small bike, ride it for a couple of years, then sell it and recoup almost all your $.
It's easier to learn on a lighter bike and less discouraging. Even though BB's and SB's handle differently, the techniques are similar, so once comfortable on the SB, going up to the BB is easier. Don't forget to pick up a pressure suit, good boots and shin/knee guards. And yes, big block tires are the way to go....mine stay on my DL all the time.
"You do your own thing in your own time"