Yeah, save the beams. Old wood, even softwood, is worth a few bucks or good to re-use. Barnboard (the siding) is sought after by artists and decorators in some places too, so you might want to check on that. If it's currently in style they'll pay a premium. If not, it makes excellent firewood.
I'd cable it so it can't fall onto the other buildings, then go at the roof from underneath, cutting it out in manageable segments. You might want to invest in a high-quality reciprocating saw for that, if you haven't already. Having to do it from underneath makes it harder, but not as bad as falling through the roof.
I'd work across and take down the rafters as they come free. I know of no easy way to do that except for making them fall in the direction you want.
You should also support the walls so they can't fall on the inside until you want them to. I've had a small, non-supporting wall tip over on me and it left a bruise. A supporting wall for a barn would do a lot more damage than that. Your wife will be angry if she has to plan your funeral and finish tearing down the barn at the same time.