I'll chime in, hoping that the OP is still watching this thread. The main thing about randonneuring which I've enjoyed is riding my bike up the road. When randoing, one gets to do a lot more of that, usually seeing and experiencing things which are interesting and novel. So for me it's the riding. And nothing beats a modern bike for getting up the road. They don't have to be expensive. $1000-$1500 will get you an all-carbon bike that's entirely suitable for randonneuring as long as you are riding on pavement or hard-pack gravel. From an entry-level road bike on up, basically for every $1000 you shell out, you save one pound in bike weight. You're not going to care about that. What works best for rando is a stiff bike that handles well and converts your pedal pressure directly into motion. All the rest is a lot of palaver.
The only thing special you're going to want will be a bigger cassette and thus a longer chain than the usual road bike. A big cassette will save your legs. A 50-34 crankset coupled with an 11-34 cassette will get you up most anything, even with a few hundred kilometers in your legs. That's another reason to buy new.
As far as vibration, comfort, and all that, it's mostly a question of tires - and a carbon frame. I think every frame made will take 25mm tires, which is all you need. I've always ridden 23mm tires and quite happily. I ride them out of choice.
Performance Bike and Bike Nashbar have several bikes in this price range. I'm sure there are other sellers as well. You might be able to get one delivered to a local Performance Bike or other shop along with a cassette change at low cost.
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