The Kona-Hoss is an awesome bike! A few years ago (before the Clydesdale forum) we had a guy on here by the name PoweredByTRD who was on his way up towards 500lbs and he got one of them and it held him just fine. I actually considered a Hoss until I got a really god deal on my Giant Rainier.
Your weight loss goal sounds possible, but it will likely take a big change in the way you eat to reach it by Christmas. I commuted 25 miles a day for over a year and never really lost much weight (25lbs or so) until I started watching what I ate. then the pounds came melting off. I'm at 280 now, which is 80lbs less than when I first weighed myself, which is probably 20lbs less than I weighed at my heaviest (I was too afraid to step on a scale).
Good luck, and check out the posts and blog of Tom Stormcrow if you need inspiration.