a fully rigid hardtail is capable of riding DH. the bigger question is how fast you want to go.
don't be fooled, SMJ. we ride to the top of ski hills and then descend all the time around here. well, at least those of us who enjoy that climb do. and we do it on light XC bikes.
if you are interested in finding out what gravity-oriented DH riding is about, I would suggest taking a day of using your own bike first, riding slow enough to feel in control, and then maybe halfway through that day, see about rental of a more cushy ride.
when you rent a bike, be prepared to think about a few things...
the heavier the bike, the harder to pedal uphill and maneuver BUT the more stable at speed and in the air.
the more suspension travel you bring to the ride, the more inefficient the uphill part or any other part during which you have to pedal.
more travel lets you go faster with less discomfort. in this situation, you probably will want more padding than your basic bicycle helmet. when I do DH riding I wear a full-face and a full torso armor suit, as well as leg armor. not even this amount of protective gear is a guarantee -- I have been wearing full armor and still been hurt fairly badly. but for sure, it helps if you take a digger in a rocky field.
don't be put off by the appearance of a bunch of yayhoos smirking in your direction with full suspension bikes and body armor. they're no different from the Gomers who buy the latest ski gear and sit around the base lift without ever doing a run (which would show their disconnect between skill and gear).