All tents have condensation under some conditions - even top end double walled ones. In conditions with lots of rain, a *well-ventillated* tarp is as dry as anything else, and drier than some. One advantage of a tarp with a separate groundsheet is you can vary the ventillation for the conditions you are facing. Also, the exposed ground will soak up drips, whereas an attached floor keeps them handy so they can run downhill to your sleeping bag.
Regarding down and wet conditions: I've seen lots of newbies thruhiking with down bags - despite total ignorance about how to care for them. I've never seen anyone wet out a quality down bag to the point that it was useless. Not saying it can't happen, but I use down bags on river trips now instead of my (several) poly-fill bags.
I think the *main threat* to a down bag is washing it. I suspect that soap residue compromises the natural water resistance of the down. Having used down bags for as long as 9 months at a time without washing them, I can truthfully say they may start to smell like a warm puppy, but rarely do they become so totally disgusting that a good airing won't fix it. This is assuming you don't have a problem with incontenance.