Welcome to touring and to the board. Now's the time to start planning long trips for next summer, so you've already got a head up on being prepared. I'm just in the dreaming stage...
These are my experiences -- and my opinions, you can surely find someone else with different views. For a cross-country trip, I don't see how you could do it using only rear panniers. You're looking at 3 months or so (which puts you before or after the height of summer), plus high elevations in the Rockies. This alone means you gotta carry a minimum of cold weather gear (hat, gloves, wool socks, fleece tights, extra jacket). I'm assuming you'll carry a tent, sleeping bag, camping gear; plus basic tools/spokes/etc. They don't call Montana "Big Sky Country" for nothing. What I'm getting at is this: you'll be self-supporting, which means at least one pannier for the gear I mentioned.
I went through the Big Hole valley in western Montana in the last week of July 2001, and it snowed. Didn't stick to the road, and it was a beautiful, memorable ride -- but it wouldn't have been if I hadn't been prepared.
If you do carry all this gear, and you try to put it all on the back, you'll probably find the bike squirrely. Front panniers are needed to balance the load at a certain point. My wife was riding a mtn bike with rear panniers only on our 2001 trip; I carried all the camping and bike gear to keep her load lighter, but she still had problems with shimmy. On the other hand, my bike (front and rear panniers) was solid as a rock. I love that feeling -- like riding on rails.
One thing I realized, after a week or two on the road, you've collected maps and souvenirs and mementos. Not enough to weigh you down, but we wouldn't have had room if we didn't have my front panniers, which were about 90% full when we started off. Stuffing everything into one set of panniers wouldn't give you that option. (Every two weeks, we mailed a box home with our souvenir accumulation, so we could start again....)
You can get clamps for the front fork for low-riders. You still need eyelets at the dropout for solid mounting, but the clamps will fix them at the upper point. If you have a suspension fork, they make mounts for those, too -- do a google search.
I have a set of basic REI panniers from about 15 years ago. My wife has Nashbar mtn bike panniers (they were on sale last week for $18 !!!!). I realize they won't last as long as the top-of-the-line expensive ones, but they've never failed us. You don't have to spend a lot to get good value.
There are good things about disk brakes, I'm sure. I can see where they'd be an advantage in downhill mtn bike racing -- not necessarily for leisurely touring. My bike has cantilevers; my wife has V-brakes. I've never had problems with them, in thousands of miles of riding, or loaded touring in the Rockies. So if you're considering switching out V-brakes for disk, I wouldn't bother. Just make sure you maintain your brakes. On long, loaded downhills, we'd stop every 10 minutes or so to let the rims cool, but I admit to being overcautious. After all, it was a sightseeing ride, we got points for stopping
-- Mark