The tips for the gear is right on: you would find it really difficult to complete an event such as the RAAM on a fixie. Most individuals who do these types of tours have bikes that have 30+ gears.
I have a Specialized Roubaix SL Pro SRAM frameset, but I re-outfitted it at the shop to have a triple up front (originally came comp) and 10 in the back (originally came with 7). Having those extra gears make all the difference when I'm tackling mountains in the various events I participate it. However, my rides are only 100-150 miles in one day, so having a light frame is perfectly fine.
Once you get the bike, you need to start training. The hardest part will be staying in the saddle for that length of time - a cross-country tour will take, like you said, 18 months, and you figure you'll be in the saddle, pedaling, for a good 8+ hours per day. While the Century rides mentioned here are good practice, they are also deceptive - imagine doing that ride again and again and again (with slight variations, of course), day after day.
So what I would suggest is continue doing your 100K rides, but see if you can complete those rides each day, not sparse them over 2 days. Slowly, increase your mileage each time until you get close to 200K and when that happens, you'll be in the home stretch.
I would also second calibro's suggestion of taking a Spin/cycling class at your local gym/health club/health spa. Be religious about going - Spinning, unlike cycling, is significantly more difficult. I always warn my class to be sure they are fitted correctly before class and to let me know if anything doesn't feel right. A piece of advice - the best instructors are the ones who ride outdoors, since they understand how to train for different terrains and can better translate bringing that ride indoors and make fun as well as challenging.
I would suggest buying cycling shoes (you can get them anywhere, however, I would suggest trying them out at the local bike shop and then bargain hunting online) and some nice, padded shorts. If you're in possession of a, ahem, life raft, you might want to consider a bib (I prefer knicker bibs) since they come up over your shoulders, pull everything in and don't pinch your stomach. I prefer bibs since I don't want the riders behind me in a paceline looking at my crack (that's honestly the last thing I'd want to see anyhow). If you already have these things, bravo! If not, now's a good time to invest.
Not to rag on it further, but swimbikerun has a good point. At 6-1 and 240, you've got a lot to pull. Weight is your number one enemy on the bike. The terrain is not always smooth and you might find yourself climbing some pretty tough climbs. The less weight on the bike, the better. (Now, weight isn't necessarily a bad thing. Aluminum and Steel bikes are the best for ultra tours since they absorb shock and vibrations the best and give you the greatest stability due to their weight. I certainly wouldn't take Reuben on an ultra tour - he's purely for (relatively) short charity events).
When you get yourself physically acclimated to the rigours of the road, it's time to discuss nutrition. You should start experimenting with what feels good to eat when you're pedaling. Those energy blocks and shot things? Those are good for refueling when you've depleted your Glc (glucose) levels and your liver is tapping into its stores of glycogen to fuel itself. When I'm doing events, I'm usually consuming a packet an hour after the first 2 hours because I'm trying (read: slowly getting dropped consecutively) to keep up with various pacelines. Carbs become your friend at this point since they provide long-term energy. After you've gotten yourself warmed up, you're going to find that you're constantly hungry. Pack those protein bars - you'll need them. It's not uncommon for cyclists to burn close to, if not more than, 1 KCal each hour while on an intense ride. For water, you should be consuming at least one bottle an hour if you're really gunning it. If not, one every 2 should suffice. I usually try to avoid Gatorade since it's not really good for you. I'll look and see what I use - let me know if you want know what it's called.
Ok - I apparently have no brevity so I'll stop here. Cycling/Spinning are my passion and once you get me rolling, it's hard to stop