serious risk of injury or death? If you arent careful and ride in front of a logging truck you will get squashed. If you ride off the edge of the road off a cliff you could be seriously injured. If you use common sense and look at the route beforehand to see where towns and or campgrounds are, you will be able to plan the days more or less, and you will have fun.
If the craigslist bikes are crap, stuff might break, if they arent stuff might not break. Break or not, you probably will have fun, you're in your 20s, have fun.

not sure what you are looking for for answers. Ride off on a trip totally unprepared on a complete craphole bike and you might not have a great time if the inconvienence or cost of breakdowns comes into play. Get the bikes checked over at least beforehand.
Or dont.
Figure out where campgrounds are. Or not.
Its up to you what prep you do, but if the bikes are at all in reasonable shape, you'll have fun.

dont spend time on the interwebz, get yer bikes bought and checked out, make sure the tent has poles before you leave and figure out if the amount of crap you will take is too much. Then enjoy the trip.